I don't blame you for being angry, I would be too if a groomer did something I hadn't asked for. It will grow back, and at his age it was probably mostly fluffy puppy coat, so his adult coat should start to come through soon. Eventually you should be able to hand strip it or use thinning scissors, but it will take a few months to grow out.
If you're intending to use the same groomer again it's probably best to give her some feedback now rather than when you book Alec in again. I think that many groomers just expect Cocker owners to have their dogs clipped all over, and that's what they're taught too. Not many do proper hand stripping.
Welcome to COL by the way

and it would be lovely to see some photos of Alec and to hear some more about him.