This sort of depends on your relationship with both people. My husband has had little crushes over the years on a few of my friends, and even one of my sisters. I don't blame him - they have been beautiful people, both inside and out. However, I know without a doubt that he loves ME, and he isn't the type that is likely to run around. The women I choose to call friends (including my sister) are likewise not the "type" to betray a friend. I trust them all, so I don't have any worries. However, not everyone is trustworthy.
I guess this all comes down to the quality of the people you associate with.
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?