I might not be that happy that the friend had left my child with her dad, but that being said, he had gone to the pub with them, and is her children's grandfather, so presumably she is used to leaving her own children with him and probably just automatically left your ds with him as she didn't think it would be a problem iyswim?
With regards to your ds talking to a stranger, tbh I would be glad that someone had seen my ds and had at least asked if he was ok and stayed with him until he saw your friend, a lot of people wouldn't bother to do that for fear of recrimination, so I say good on the guy. Whilst I do agree that children should be taught that they shouldn't just talk to strangers, they do also need to learn that if they're lost or in trouble, then they should feel they can ask for help, and that sometimes, asking for help means talking to a stranger. Yes we do need to be aware that not all strangers are nice people, but stranger danger is not nearly as bad as the media would have us believe, children are at greater risk from those they know and trust.
You really should try not to worry, nothing happened, and in reality your ds never was in any danger, he could just as easily have wandered off/lost sight of you in a pub garden, yesm moving tables was a bit thoughtless, but really he wasn't ever at any risk.
Also, if your ds is a worrier, then you worrying could potentially make that worse.