A really fucked up thing a thief of time could do would be to steal the time it would take for a friendship to corrode. Suddenly two people go from being the best pals to being really jaded and annoyed with each other

I feel a Heart or Blood Player would work better for that, since it feels like, having it deteriorate over time would only happen if they were already predisposed to have their Friendship go that route. Otherwise, true friends, no matter how much time passes, would keep sticking together.