A 10 gallon for a betta is a great idea.

Please don't advise him to get a 1 gallon if he's willing to spring for a 10.
It won't be maitanence free, obviously, as fish keeping will always require weekly or daily work.
A 10 gallon planted will create a environment that's incredibly stable, and very simple to maintain. I pretty much have that going on for my female betta. She's very happy in there, with something like 80% of the tank being planted.
If you do feel the need to bring down the tank size, I wouldn't go for anything less than a 5 gallon for a 'low maitanence' tank. Still big enough for a heater, still big enough for him to swim, still considered 'small', and only 8" bigger than a 1 gallon drum style bowl.
I have nothing against 1 gallon bowls mind. I kept my betta in a 1 gallon bowl for a year. But I would have to say that the 5 gallon he's in now is definately a superior home for him, and I dont' think my female betta could ever be happy in a 1 gallon home -- she's got too much wunderlust.
