Such a quandary for their quilt, when what it symbolizes is so important. You didn't mention if your friends are guys or gals. That would make a difference in the quilt you choose to do.
My feeling is that rainbow colors would look great in prints with the same tonal qualities (but not pastels) against a printed background of either near black or cream.
If your friends are guys they might like this pattern called Star Weaving, from Marcia Hohn:
http://www.quilterscache.com/S/StarW...ock_Page3.html
And if they are gals, how about a pattern Marcia Hohn calls Golden Wedding Ring (different than the traditional GWR):
http://www.quilterscache.com/G/Golde...RingBlock.html