Me and my wife have regularly sex relationship and as know we have to perform the big ghusl before we can fast and pray. My wife has long hair and can’t afford taking a shower and washing her hair every day. Is it permissible that she only wipes her hands with water on her hair or she have to take the shower?

It’s not allowed to remain unwashed once hair during Ghusl e Janabat( major ghusl). She/he must wash whole body.But there are many ways to make dry once hair by using dryer or at rapping with towel etc.