It's the problem/issue for the married person - they're the one that made the vows - not the responsibility of the other person. Sure, it depends on that person's morals/beliefs, but it's down to the person wearing the ring to worry about the marriage. They're the one in it.
NO,NO and YES (except as a friend,put your self in the wife position (metaphorically not actually ,that was a no!) and ask yourself how you would react. be guided by that.
Unless of course you plan to tell her and your husband and he is ok with it, and if you were not, ask yourself why not,guilty?
There is nothing wrong with seeing a married person. You have not made any vows to their partner, it is them that is cheating not you. No one held a gun to their heads and forced them to cheat. My feeling is if they are cheating, then the marriage is not up to much anyway. I was seeing a married man for about 6 years and loved every minute of it. I never felt guilty about it as it was him that was cheating. I saw him for all the good stuff but still had my freedom. It was great! My partner now is also married but he is separated and we live together as a couple.