I am assuming that "brake light" is a reference to the brake warning light on the instrument panel, rather than the brake lights on the rear of the car. If that is the case, the first thing to check is the level of the brake fluid in the master cylinder.
If the brake fluid is low, replenish it with the correct-spec fluid.
Then, have the brake system checked. Because brake fluid does not evaporate, a low fluid level indicates that:
There is a leak in the brake hydraulic system
The brake pads are worn. As the brake pads wear, the brake calipers extend further and draw additional fluid from the master cylinder.
In either event, brake failure is in the offing for this car, and that is about as serious a situation as can exist with a car.
His brake system needs attention, and this is not something that can be safely deferred.
Tell your friend to refuse to ride in the car and to refuse cooperating with her boyfriend in other ways until he has the brakes attended to.
If you don't know what I am referring to when I say "refuse cooperating with her boyfriend", a quick read of a synopsis of the ancient Greek play, Lysistrata will give you the essential information.