I agree with the previous answers regarding nutrition, hydration and foam rolling and would like to add that I had a client who tried everything and still got leg cramps on those long miles. He started wearing full length compression running tights and no more cramps! Compression socks would probably work just as well for your client if he is only getting the cramps in his lower legs. It is worth a try! He might want to try taking more salt during the long runs too. I find that for half marathon distance I am fine with regular electrolyte replacement drinks but once I get up into those longer marathon distances during training (and races) I need more salt than is in any of those drinks. I take those little sachets of salt with me and pour half into my sports drink at about mile 12 and then the other half at about 18-20. Some people I know just eat the salt right out of the packet and that works too for those who can stomach it!