Probibly flesh injories. Puffers aren't community fish and are likely to have attacked him/her. Cince elephant noses are scaless, they will have done a fair amount of damage, even if they are small. I'd advise you mum to re-home the elephant nose to a 75gal with at least five other elephants as they are a shoaling fish.
Slime or fungus is also a posiblity, but injories as a result of puffer attack is more likely IMO.
HTH
Rabbut