In my opinion, if the outfielder makes the call soon enough and loud enough then any collision is the fault of the infielder. Otherwise I would blame the outfielder.
That said, I acknowledge that the may be close situations where neither is at fault or both are at fault.
I am praying that both players recover quickly and suffer no lasting consequences, and that all of the players learn from this. As I recall, Jason Kublel blew out a knee from colliding with another outfielder and was never the same. I am still sad about that.