I agree. If he doesn't get positive talks about potty training....and always yelled at for having an accident this could be his fear. To get into trouble. I'd be concerned.
Honestly this ONE time I wouldn't tell the parents jsut becuse I want this child to TRUST me and be able to come to me honestly with any issues. If I tell his parents after I already "helped" hide the mess I fell like I'd break his trust. *IF* it happened again then I would consider telling the parents....but it's just a one-off then no I wouldn't.
You've already helped him cover up since you washed his undies.