My friend just got rid of her hot tub. The person that took it put an outlet where the tub was. He put in a 15 amp GFCI at the outlet but left the 50 amp GFCI breaker in the panel box.
Is this sufficient or trouble waiting to happen?
This is trouble, but easily solved. Simply swap out the 50 ampere GFCI breaker, for a 15 ampere GFCI breaker.
You'll possibly have to use pigtails to connect to the breaker, as it may not accept the size wire used for the existing circuit.
As "subpanels" seem to be quite popular around here, I'm surprised it hasn't been suggested yet. You could always install a second panel where the tub used to be, then install a 15 or 20 ampere breaker to feed the receptacle.
You'd also need to change the breaker, because the 50A circuit is probably 240V. You wouldn't want to simply connect those wires to a receptacle, unless it's a NEMA 6 type.
In any case you need to use a 15A breaker (or a 20A breaker if there are 2 or more receptacles, or you use a 20A receptacle which is downward compatible with 15A plugs).
It's possible the hot tub was connected 120/240 with a neutral. If that's the case, you can use one hot and the neutral just fine. In fact, you can re-use the neutral with the other hot, and get 2 full circuits. That's called a multi-wire branch circuit.