Was this appropriate?

From what little we know, and indeed what little the OP overheard, I find it rather strange that the GF is taking so much flack - seems to me that the conversation between the daughters, which ended with the 'boobs man' comment, is equally likely to have been an 'inappropriate' conversation, especially in public.
Moreover, if I have understood correctly, the daughter who threw in the comment (the boy's mother?) didn't show any signs of having been upset by it, did she?
If I were you, OP, I would have light-heartedly raised the topic afterwards with your daughter, because she is probably in a much better position to know (from whatever her son had said to her) whether anything genuinely inappropriate had been said or not, instead of all this speculation.

Good past MamaCaz. I agree.

I also think that an off the cuff comment, repeated 3 times which no else has taken offence at, should really be dismissed. I can understand you OH going off in a huff and feeling defensive, this is surely not worth having an argument about??

If he’s said something to his daughter or grandson he’s a sleazy undignified pathetic old man. Ok, the grandsons heard far far worse from other lads, but for his grandfather to engage in such a conversation is inappropriate. I don’t think you are going overboard at all, I’d have gone mental!

80 posts and nobody knows what was said.......

And now it’s on Facebook, I hope the OP doesn’t get outed

The fact that it’s on Facebook is no different from it being on here, Mary.

Anyone can see the posts on here.

Anyone can see the posts on FB, but only if they already follow GN on FB or they search for GN on FB.

As we are not sure what exactly was said, or in what context, to call the grandad a sleazeball is a bit harsh! Grandson may have heard talk about males fancying 'boobs' 'bums' 'legs' etc from other lads, on TV etc. & needed to check it out for authenticity. He may have asked his grandad what his preference was. Might be totally innocent! Poor grandad my have been caught on the hop and just answered honestly!
We can always be wise after an event or be judgemental when we are not directly involved. It is a shame if GD did not clarify the superficiality of physical attraction but we all miss golden opportunities.