It depends on his stove! Mine takes a long time to come to a boil with 7 gallons, but then it boils perfectly. Some people can't boil two gallons of water on their stove. So, see if you can figure out how much water can come to a boil on his stove before investing in a pot (or go ahead and get the burner- which might be a waste if he doesn't take to brewing).
I wouldn't bother with a 5 gallon pot, though, unless you know that he can use it for something else. It's big enough for small batches, or for partial boils, but it's not really big enough for a 5 gallon batch. Yet it's too big for boiling corn and using in the kitchen.