Sorry, I should clear up what my understanding is:

OP tells his GP surgery during the appointment that he did not want letters passing on. It is possible that from this point he is referred to an IAPT service (more than likely). It is probable that this was not passed from GP > service (as they are from two different providers).

If he has made a request to NOT have written communication and this was received, but then done otherwise, then this would be partially the responsibility of the organisation sending the letters out.

If OP does not live with his mum anymore, there is a chance that he did not update his information with the GP (or this information was not sent to IAPT).

Hope this clears up some confusion, I admittely was a little too brief in the original post which I don't think reflects well for the sub.

If the information mishap is on the behalf of the GP/counselling service (which if on a NHS trust may have their own records which sometimes coincide with the GP, but require reconcillation on a manual basis though would show a discrepancy), then he would need to contact a practice manager or PALS if this is another service that isn't done through the GP.