wow I would say that you are not being unreasonable at all
He's going to stay with an old school mate -eh?
What if there is only one spare room - if he has a two bed apartment / house - is your Dd going to share a bed with her dad? what is the iteneary - Oz is HUGE!
And he's gone into debt? eh? in an economic downturn? To fly across the world to see someone who he hasn't seen in 7 years.. will he have enough to cover an emergency?
Blimey - it's a very long flight back if your XP decides he can't cope again!!!
YANBU.
Besides what's all this nonsense about this trip being once in a lifetime folks? The OP's DD may well be able to pop around the world every month when she's older. Why are you limiting her future, just because her dad's gone into debt to go when she's just 8.