If they've got that much free time, they'd be better off using it for exercise or something.
If they've got time to do that kind of thing, they should use it to exercise instead.
If they've got that much free time = それほど暇があるなら
〜 or something = 〜とか、〜でも
that kind of thing = そんなこと、そういうこと
be better off = 〜をした方がいい、もっといい状態になる、もっといい結果になる
アメリカ合衆国 
