Good points above about whether he'll use it. But since his doctor suggested a trainer, there's at least that motivation.

I'd ask the brother to ask the doctor for some guidelines: how much time riding per session? How many sessions per week? What intensity level?

For the last question, if the brother will be using a conventional trainer without smart software features, a pulsemeter/heart monitor would be a worthwhile purchase.

Also, for motivation, there are plenty of workout videos on the Global Cycling Network (GCN) YouTube channel and other channels with a bunch of riders and a sessiion leader that he can work out to.

Edit: some examples of indoor cycling workout videos.