I suppose that's always a problem, how to give your alies help, but not take over their land. I suppose it's possible to sit it out a little bit, until you get a peasant rebelion instead of a loyalist, and then hope your ally goes for it and takes it before an enemy. Or, if an enemy does take it, that your ally can take it from that enemy.

It's always kind of a pain. It'd be nice if you could just sell off the land to someone.