All he needs to do is buy a small resin bottle (you can buy them pre filled with resin). He doesn't really need a tank; a 25 litre water barrel is enough to rinse one car.
Just fill up the 25 litre water barrel, pour it - bit by bit into a bucket as he uses it - and rinse the car off with a small plastic jug.
It's how I do it (on the very rare occasion I wash my car), and when it dries it comes up gleaming.
Tell him to also get some hose and a cheap water fed pole, and he can sack his window cleaner and clean his own windows too. (First make sure that I'm not your mates window cleaner; ask him if his window cleaner is a good lookin' Geordie Jap (there's not too many of us about this time of year)).