You can either cast Tragic Slip in response to activate Tree's ability, or the other way round. The latter will obviously be too late for anything interesting to happen, as the exchange already happened.
If you do the former, Tragic Slip will put Tree into your graveyard, and when the then-resolving ability tries to exchange things, it doesn't find an object to exchange toughness with, as Tree changed zones by then. Here's the relevant rules quote:
701.8a. A spell or ability may instruct players to exchange something (for example, life totals or control of two permanents) as part of its resolution. When such a spell or ability resolves, if the entire exchange cant be completed, no part of the exchange occurs.
Example: If a spell attempts to exchange control of two target creatures but one of those creatures is destroyed before the spell resolves, the spell does nothing to the other creature.
June 30, 2016 10:15 a.m.
I think he was hoping it'd use the Tree's last known information and see it had zero toughness when it was put into a graveyard.
June 30, 2016 10:56 a.m.
9/22/2011: If Tree of Redemption isnt on the battlefield when the ability resolves, the exchange cant happen and the ability will have no effect.