Mod/meta notes:

Most of the time, we want OPs to be in the woman's words, while they reveal the consequences they suffer due to their choices to ride the Cock Carousel and put off marriage until they've "had their fun".

However, much like this one from a few years ago, this one is exceptional. And unlike the one from a few years ago, this one is funny!

Why bother with single moms in the first place?

Unless she keeps her children completely separate from you, why would you bother?

Most of them are looking for help and free labor.

The OP leaves out way too much info. Was he smashing?

That changes everything. I'm not saying that if he was smashing that would make it ok. It wouldn't. But for the love of christ, if he was not even smashing, wtf.

His first mistake is being friends with a single mother (especially one with such young kids). Even ones who aren't deliberately trying to use you end up doing so, because they are cesspools of need. Raising two kids is hard enough with two parents, let alone one. Maybe he was friends with her before she had kids, I don't know, but regardless, that's just a dangerous situation.

This is true even if, up to now, she managed to not ask him for much. Single mothers have very little reserve left, be it energy, mental capacity, money, support, etc. if you're around them long enough, they will have a meltdown or breakdown of some sort when a stressor arrives (she loses her job, the kids act up, baby daddy drama, whatever) that she can't handle alone, and you'll feel obligated to help.

This is what I suspect happened to the OP. He might not have been helping her much before. Maybe she managed to keep her single mother needs under control in their friend group. But it's clear she was hitting a breaking point when she's saying she needs a break and a vacation. That should have been a warning sign. People go on vacations to take a break from a lot of things. 9 times out of 10 a single mother is looking for a break from her kids.

Finally a few notes about vacationing with young kids, for those among us who are childless :) it's been said that vacationing with young kids is just taking care of them in a different place, one without all your shit. It can sometimes feel like that. You're left wondering if the sights and sounds of the destination is worth the fact that, even after stuffing multiple suitcases full of their stuff, it's still not as easy to take care of the kids as if you were home. A two bedroom suite is nice but it's probably still not as big as her house and certainly doesn't have everything setup for their daily routine like their house would be. This means taking care of them is usually harder than being at home.

Secondly, families with nannies often travel with their nannies, precisely because otherwise it doesn't feel like a vacation, just taking care of the kids at a different place. Here's what it costs: you must pay all of her costs, including a separate hotel room (unless you're airbnb'ing a large house where she can have her own room), plus give her set times when she can be alone and not taking care of the kids (usually set times each day, plus like a full day off if it's a week long vacation), plus pay her her regular salary for that time (sometimes more like 1.5 since she's away from her own family for this time). That's the baseline, and sometimes you end up paying even more, like giving her some spending money (per diem, even after you've covered all her meals and lodging), and of course, bringing her on any excursions, day trips, etc that you may have planned.

It adds up fast (both the inconveniences of not being home, and the expenses of childcare) and at least once during the trip, you'll wonder why you didn't just do a staycation at home :)

If he knew all of this beforehand, he would have gone running. I'm glad he eventually wised up and ran.

(That said I have to wonder. He's a single 26 yo guy. When I was his age, one of my goals in vacation was to hook up with girls, either locals or tourists letting their hair down. Even if this friend of his didn't have kids, why would you vacation with her, and have her cockblock you all week? Even when I was having sex with someone regularly, I usually preferred vacationing with guy friends as it was a lot more fun to go out and flirt with new women, or overall let loose, than spending the whole week in a hotel room with the same woman, who's expecting you to do couples stuff all week).

Ah, to be young and naive again. The joys of letting people use you because you think it's what you're "supposed to do."

Glad the guy grew a sack and got some value out of the trip.

In my pre-RP days I met a single mother on OKCupid and went on three dates with her. Our first two dates were lame and there was absolutely no chemistry, but she seemed nice, so I was willing to give things a chance. The third date was with her 2 offspring at a kids' science museum. I liked the kids and there were no "problems," but the whole situation seemed off; again, there was no chemistry, and I wondered what my motivation to be in the relationship was.

Never again.

Even acknowledging that women take any opportunity to use men as heavily as they can get away with, it's interesting how it got to the point of surprise-offloading her kids onto a friend while on a trip so she can have the vacation to herself. It's quite the Hail Mary, so surely their friendship dynamic already had a history of tolerance for this kind of thing, which perhaps explains why the guy rolled with it the first day: he genuinely doesn't mind helping from time to time, even if he realizes there isn't an actual reason for him to do it.

The day that follows is very typical woman behavior: instead of being thankful for the effort someone else put in improving her life at no benefit to him, she ponders, can she get even more out of him? It's the same base thought process that fuels hypergamy, an unrestrained need to maximize her quality of life at the expense of others. All women suffer from this, but single mothers more so because they don't see their fatherless kids as a consequence of her own bad life choices, but rather a justification for the world owing her something. That's why a relationship or even a friendship with a single mom is a waste of time, they have no value beyond being used because they'll just use you whichever way they can.

"I (26M) have a close friend "Sarah" (27F) who has two kids, ages 3 and 5."

Why?

His mistake was being "close friends" with a woman in the first place.