First of all, no matter what, if you know about it, you should confront her.
After confronting her you can either:
A: Find out what she has to say for herself and choose to forgive her and try to make things work.
B: Not accept whatever she says, and break it off with her right then and there.
OR
You could try to sweep the information under the rug and forget about it, but one way or the other, somewhere down the line, things will go south either way.
I personally would do, and have done the other things I suggested above. Sometimes "A" works, and sometimes it doesn't.
Ultimately it comes down to how serious you are with the girl you're with.
If it's pretty serious I'd say it's worth trying to salvage, but that's me.