Part of this can get really tricky.
If you're by yourself and you only use your equipment 10-15% to them would be more than fair,simple finders fee is good.
If you're using only his equipment I think 40-50% is fair.
If you're using a combination of both, I would work with each other to come up on a fair rate. Say you need more extraction power but have enough equipment. He rents the machine out for x per hour, or x per day. Same thing with drying equipment dehus and airmovers x per day.
Another way is you bill the entire loss, and whatever labor, equipment rental charges his equipment made he gets to keep. That is a lot of tracking, not sure if it would be a big headache or not.
If he is wanting you to take care of all his clients' water damage it might be worth it looking at what it would take to buy some or all of his drying equipment off of him so that you can keep more at the end of the day.
If you're not in the position yet to invest a lot of money buying his equipment you can take the last option and work out a deal that he take half of the full rental as payment on the equipment. It all depends on what kind of business relationship you two have.
It seems to me he still wants to make money with none of the headache involved or liability.