Chemical Sraighteners

My male friend who has really nice long wavy hair thought it was a good idea to get it chemically straightened. Turns out, as I told him it was a horrible mistake. It didn't really even straighten, which was what I thought would happen. He just has very long (past shoulder length) hair and it's got a tad bit of wave to it now.

I was just wondering if there was anything to make it sort of get curly/wavy again without doing anything to it? Maybe any products, keep in mind he is a guy. He's more than likely not going to want to put a lot of crap in his hair or buy a lot of things to get his natural curls back. He's been using 2 & 1, Shampoo and Conditioner, which I told him maybe to try just using some conditioner to see if it helps any.

Maybe some deep conditioning treatments? Anything. I'm sure gel, mousse, hairspray, ect is out of the question. Sort of looking for something he can wash out. Thanks.