Here's a tip from Boy's Guide to Girls: 30 Pointers You Won't Get From Your Parents or Friends
Q: Is it okay to go out with your best friend’s girlfriend?
A: Yes, if you’re going out as friends and all three of you know what’s going on. Absolutely not if you and his girlfriend have romantic feelings for each other and you’re doing it behind your friend’s back.
Situations like this happen, but before you two act on your feelings, understand that it could ruin the friendship between you and your best friend. Most of all don’t do anything without everyone being open and honest about what’s happening. That means she needs to end her relationship with your best friend before you go out with her.
+Boy's Guide to Girls