Gaining weight after quitting smoking, despite mostly healthy lifestyle

Hello. My friend has given up smoking. However, he's gained some weight around the belly and he's talking about taking up smoking again.

He has a very active job, maybe not exercising all day, but certainly not sitting all day. He exercises every day--not a huge amount, but some. And he doesn't really eat a huge amount.

Any ideas to help him take this weight off? I don't know what other options he has if he's fairly active and doesn't eat much.