I slept with a married woman that was having troubles in her marriage. Fast forward 2 years and now my current girlfriend has became really close friends with her. She doesn't know.
Are you planning on keeping it a secret from your current girlfriend?
I don't know if I should tell her or not.
It's not good to have an affair with someone who is still married, even if she isn't happy in that marriage, but that was in the past.  The best thing that you can do is be open and honest with your current girlfriend.  As long as you don't still have feelings for the other woman, all can be forgiven.
You're right. I will do it as soon as I get the chance.
That's a good idea, because you wouldn't want your current girlfriend to find out from someone else. Of course it wasn't appropriate to get into a relationship with a married woman, but lying to your girlfriend about it would also be bad.