General

It's cheating.

It's up to you where you place your moral standards. Hers seem pretty low.

If the partner was aware and totally cool with it, all good! But either that's not the case, or she's lying to make you feel more romantically involved.

My advice is to pass. You might feel comfortable with her, but you have no idea what her partner is like. Take it from someone who has had some experience with unstable people. Meth addict. Crack addict. You never know what you might run into.

It's a very hard situation, but if your uncomfortable with the relationship, then you should end it.

I'm not sure what you are looking for but I doubt guilt was on your list

are you looking for a serious rel or just an exp? if you are looking for the former you did well but mind you it is not so easy to look for someone who shares your interests