bridge friend
Someone who is friends with two people before they engage in a relationship, and who remains friends with both people throughout the relationship (inc a breakup).

The "bridge" between the two, socially. When a problem, such as a breakup, develops between the two, the bridge friend is the one who gets all the awkward questions the two in the relationship don't have the stones to ask each other.
Adam: Dude what's up with you?
Bret: Uh, Cathy collared me and started asking a bunch of questions about Dylan and why he won't return her calls.
Adam: Ouch, sucks to be the bridge friend!
Bret: Too right!