if you can score a m50x for only $100 its not a bad deal. closed headphone with good sound quality. v-shaped with decent treble and bass. only thing it suffers from is extremely small soundstage (not much depth or distance feeling to sound) and the earcups are a little on the small size so are bad for big ears. i have the older m50 myself and its been good to me. better for music due to soundstage but you could play some casual gaming on them.
the hd518 or hd558 are good open cans in that price range. fairly flat response and typical sennheiser laid back sound. average soundstage. good general use headphone. good for anything.
ad500x or 700x if you want something more tuned for gaming with wide soundstage and bass light sound for competitive gaming. i'm thinking you want something a little flatter or fun sounding though since you're casual.
agreed that a dt770 or dt990 are nice headphones but its also suggested to have a decent amp or soundcard for them also which would raise the budget if he has none. otherwise you wouldnt get all you could out of them.
if you wanted something with a mic, the original plane jane hyper x cloud isnt bad as its based on the takstar pro 80 headphone which while not well known is a good budget can.