When you eavesdrop, you shouldn't complain about what you hear. I agree that the person doing the eavesdropping was in the wrong. Having said that, and since that horse is out of the barn already, this is a golden opportunity to see if that friendship is as nurturing to the hurt friend as she believes it to be.
IMO, friendships are relationships where both parties are connected because they complement each other. Each gives something to the other that satisfies a need in their lives whether it's companionship, sounding boards, loneliness, etc. If your friend can go a week without a conversation to her friend when they've been talking three times or more a week for awhile, perhaps they've grown in different ways and this is a friendship that needs to end regardless of how long it's been going on. There IS such a thing as being in a rut, you know.
I wasn't there and don't know what all was said, but if this had happened to me I think I'd be seriously considering backing off on or ending the relationship completely. I have little tolerance for those who would talk about me behind my back. It would make me wonder what else they're revealing for the sake of small talk (or venting) that I would have wanted to be kept in confidence.