A lot of this may come around to asking the right questions as a designer. He says he wants them to be able to climb the tree. Your questions should draw out more information. Does he want a romantic place for two lovers to cuddle, or does he want a place for puck to hide? Once you know exactly what he wants, you can either find a way to accomplish the same task with another set piece, or alter the tree to safely do what he needs it too.
I had a similar project recently and it ended up that the director wanted one actor to climb the tree and hide there to overhear other conversations. I ended up building a tree trunk, hanging a bunch of branches in the air to (disguise that the trunk disappeared into nothing), and building a small ladder in the back side of the tree so that the actor can climb the ladder and peek around the trunk. This way the director could get the moment he needed without me needing to use my whole budget building a tree with a branch that could support the weight of an actor.