Like the others, I have to wonder why/how coolant got into the oil. But since the oil
was changed for whatever reason, it does sound like the plug was loose and fell
out after your friends hit the road. If so, shame on that mechanic.
Don't say that doesn't happen. It happened to me in our old Blazer. Driving later
the same day I had the oil changed, the oil pressure dropped like a rock in nothing flat.
I got to the side of the road ASAP and had it towed. Nearly all the oil had leaked out.
Luckily there was no damage.
Please pass along my condolences to your friend.