How about I  sleep with a friend without asking her if she effectively left her boyfriend?
Are you looking for a one night stand or something a little more serious?
A one-night stand would be okay.
I think that you should make sure the relationship is really over first. You should not get involved because it could complicate matters even more.
Maybe you are right, but I want to do it.
Things get very complicated in relationships when you start relations with someone a friend has dated. It would be better to try to find someone else.