what to do when youre the "other woman"?

[loxiaxx]

So basically, my very close friend kissed a guy she liked for MONTHS back in June who told her he wasnt "too serious" with his one-year gf. They supposedly broke up then got back together but he told her she "does what she wants too"... my friend fell for this(and him obviously) and one day, before he left to work abroad for the summer, they spent the whole day together, hanging out and she said they kissed quite a bit. He asked her for sex which she refused and the next day he was gone. She said she often thought about him but was curious what would happen between him and his gf(who lives abroad).

Nearly one year on, shes found out, and my friend is petrified that the gf found out because she wrote to her on facebook "when I come to visit you, I'll kill you". My friend, being the insanely people-skilled/anxious/guilt freak that she is, doesnt know how to play it..she knows she did wrong but shes been threatening to hurt herself over the guilt when she simply wants to make "ammends" with the gf.

What should she do?