I admit, I am a little new to D&D, and even newer to GM'ing. However, I feel that the situation at hand has created a bad experience for all parties involved. There is no reason to let this player be utterly useless because of 1 bad role. However, it should be punishing...but as a GM, you have rule over HOW punishing, even if the rulebook says otherwise.
For example, you have the player roll "Religion" to speak to his or her deity, and then roll an int save to perhaps gain +1 int back. Effectively having their deity grant them consciousness again. Making them, a retard, but still able to fight and be useful. A savage warrior who's mind can only fashion together combat, and basic instincts. Perhaps other PC's have to care for his/her more...basic needs, like eating, as they might not understand the difference between edible and inedible foods.
The DC for them could be relatively low, and even if failed, can be repeated...say...once every long rest aka 8 hours, or once a day?