Avocado, I was going to say to you just to be careful that you don't befriend her on Facebook and tell your PT that your private issues are not to be related to all and sundry thanks very much.
Then I read your post of 10:26.
So, she suspects he might have been cheating throughout their 10 year marriage? Did he tell you this? You say you don't know if he had or not, did you ask him?
To my mind, a man who cheats on his wife is not worth a second glance.
Then you say that she doesn't want a divorce and is refusing to sign certain papers. They also have kids.
So here is a man who may have cheated on his wife and who is going through a messy divorce. I know you said that it was just a drink, but clearly it isn't. It's a date.
Sorry, but I would take a huge step back and ask him to contact you once the divorce is through. There's surely no way you want to get mixed up in what may be a traumatic divorce with an ex-wife who may use every tactic to get her ex back, particularly when she finds out that he is dating someone else. And yes, now she knows who you are, once she does find out that it's you he is dating, things could be uncomfortable for you. I would certainly be changing gym in case I bumped into her again!
Why make things more complicated for yourself? This man might be an old flame, but he's also possibly a cheat and in the middle of a lot of emotional baggage. Right now he should be focusing on his children and finalising his divorce, not dating other women. And you've only been separated 3 weeks.
My advice would be to cancel the date and go out with the girls instead. Make the most of being single. Get some girly DVDs and a bottle of wine in, watch what you like on TV, make plans to go to the cinema, redecorate, catch up on the friends you might have neglected whilst you were tied up. If this man is a catch, then he'll still be around. But I think to go on a date with him at this moment in time, when he has so many issues going on with his ex, would be a mistake and would make your life very much more complicated than it needs to be.