I agree with all those who say you need to know more about what he is proposing to do whilst in Oz.
Has he thought out where he would visit? Are these places suitable for children? Will he stay in every evening that he is away? What if his mate wants to go out in the evening and suggests someone else to babysit, what will he do?
I can understand why you would be reluctant to say yes to this trip but maybe if he can answer all your questions, it might put your mind at rest.
Although it is a lovely idea and might give him the chance to build an even better relationship with his daughter, I guess you need to know whether he has actually thought this through or not.