It is illegal in the UK to feed a live vertebrate to another creature. I expect it is done but if it was seen the person could be reported, and would be if I saw it.
Actually here in the UK it is legal to feed a live vertebrate to any other vertebrate or invertebrate unless unnecessary suffering and pain can be proved to have been caused. Since the fish appeared to be swimming about and totally normal until the person saw the other side of it's face, to be honest that would imply that it was unaware of the damage.
Prosecutions for this kind of thing are entirely dependent upon you being able to prove to the legal bodies involved that the animal being eaten was definitely suffering and in pain.
It is however completely illegal to feed a live vertebrate to any other organism at a time or place where it may be viewed by the public

so you'd have them on that one anyway