That's what I thought. But is that how the rules describe it?
Bull rush allows you to do a push (p. 287).
You can't push a creature into a square that it couldn't walk into (p. 286). So, presumably the converse is true: if an ally can walk into it, he can be forced into it.
A creature can walk into a square with its ally (p. 283), even though he's not allowed to stop there.
So with this line of rationale, a creature can be forced into an ally's square because he could enter it himself. But this creates a situation that is seemingly undefined in the rules.
So I would guess it's not legal. Perhaps because of the fact that you can't end movement in an ally's square.
And note that this all assumes the ally is standing up. If he's prone, there shouldn't be any problem.