TLDR: Mutual attraction with 8/10, her relationship is inevitably ending, she was faithful and is very sad. How long do I wait to make a serious move?

I'll keep the story as short as possible

There's a girl I have a lot of contact with that is definitely into me. I don't have oneitis, but she so far seems to be above-average as far as potential LTR goes. Regardless, I wouldn't date her immediately. She is very attractive, takes great care of herself, and is surprisingly fun and funny. She also comes from a lot of money (from another country) and has confidence issues (of course).

Here's the weird situation.

This girl has been dating the same guy for almost 3 years, and she just found out she's moving back to their home country in a couple months, permanently, and she can't come because she has to be in school here for another 3 years. She has already told me she knows it has to be the end of them, but it has yet to be officially ended.

So, when that day comes, when should I make a move? I've been escalating kino daily, to the point where if she was single I'd have kissed her already. She's surprisingly loyal, maybe due to her different culture; I'm pretty sure most girls would have cheated with me by now.

I've been wondering about when to make a move assuming they break up. If she tells me in private, should I immediately kiss her? Should I wait, and ask her out on more of a "date" and then escalate from there? The issue I have is I don't know how long girls tend to be sad about this stuff before they go off chasing other dudes. She was very much down in the dumps today, to a point where I know she wouldn't have wanted a kiss even if she was single.

I've rambled enough, and you guys can ask questions to clarify if you want. I'm a long time lurker but I've never read anything about this or had any problem worth asking about until now.