What size tank? Are both sides filtered & heated? Frequency of water changes & how much changed? How long has she had him? With the glass it doesn't appear there would be good water circulation if the tank were filtered & heated. Does he look bloated or like he's pine coning? If he's having trouble getting to the surface for food & air I would put him into something smaller so he can do both. You'll need to keep it warm & change the water probably daily. If his current tank is heated you can float the container in the tank, if not you'll need to figure out a way to heat it. if you float it I would make it a non see through to see if it makes any difference that he can't see what's on the other side of the glass. From the pic you posted it doesn't look like he has much cover from plants real or silk & that can cause stress to a Betta.
