I think it depends on the environment and the people that you're talking with. Especially for men, it is almost a casual way to salute each other like this. But of course, if you are talking to a really conservative and religious person it might be a problem ( personally, i think it'd be overreacting though) Besides the religious meaning, it is also part of the language now. Anyway, he can go with the phrase "Selam", it is easy to pronounce and a casual word. Good luck :)