Who instigated things between person A and C? If it's person C then I think they are in the wrong, trying to get someone you know is in a relationship into bed is a big no no in my book
If person A came on to person C then I don't see that person C is to blame. Person A and B are responsible for maintaining their own monogamy, why should person C be even partly responsible for maintaining the monogamy in a relationship where at least one party doesn't seem to value it?
The other thing to consider is do I, as person C , know person B? If so then out of respect for my friendship with them, I wouldn't go there.
At the end of the he day though, we're all going to have different answers based on our own moral compass and experience. Who am I to assume I have the only right answer, and thus all others are wrong? The above is what I believe to be morally right, what would allow me to sleep soundly at night. Others will have other opinions, so long as they can live with themselves it is not for me to judge them.