I reckon the owner should get it checked out asap.
Years ago one of our foals leg joints started swelling and it was due to his umbilical cord not healing over properly or something, hard to remember clearly as was about 12years ago! can barely remember last week *lol*
The stifles went first then the knees swelled up, thankfully we got there in time and he had what he needed form the vet, but it was a life threatening condition, dont want to scare you, but i would get the vet out, it could be something simpler than an infection caused through the umbilical cord, but you never know. Our foaley seemed fine in himself too,they tend to go down hill all of a sudden foaleys from my experience.