Yes as Michael says anyone can witness a signature on an EPA or a Will for that matter, as they are merely acting as a witness to the signature, not to the contents of the EPA/Will, or necessarily to the mental capacity of the person executing EPA/Will.
When people are signing wills in hospital, nurses are not allowed by the Health Authority to act as a witness, as with their medical knowledge their signature could be interpreted as confirmation that the person is of testamentary capacity. Admin staff were sometimes OK to sign but there seems to be more reluctance from the hospital to allow them to do so.
The new LPA's will require someone to confirm that the Donor had the metal capacity to sign. The proposed list of competent people to give this certificate includes doctors, solicitors....shop keepers
(no offence meant to any shop keepers but not sure where the government is going with this one)
If anyone is concerned that their mental capacity to sign an EPA/Will may be questioned in the future, it is worth asking the GP to examine them and prepare a short report.
My grandmother's testamentary capacity was (briefly) questioned by relatives who discovered that they had been excluded from the Will. My mum was able to confirm that on the afternoon she signed the will, before the Solicitor came to the house, my grandmother and I were playing Snakes and Ladders together and we were BOTH cheating. I was 10 and she was 75...
Sue