My guess is that Friendly's use of that phrase dates back to before the days that it was a well-known wink-and-a-nudge phrase related to Asian massage parlors.
https://english.stackexchange.com/questions/294866/when-did-...
I'm not sure there's a clear boundary between the usages where the second one just jumped out of nowhere.
Someone in the thread linked above mentions lyrics from 1978 that mention a "happy ending":
https://www.azlyrics.com/lyrics/olivianewtonjohn/alittlemore...
You can point to an authoritative source like the OED, and at the same time question whether it really penetrated to everyone.
Considering that, as I wrote, it was used until 2018 by one restaurant chain, that might be a plausible estimate of when it became commonly known to most people.
So is it reasonable to come down like a ton of bricks on someone in 2021? Is it fair to say that 19 years is not enough time to adjust, while 22 years is more than enough?
I am expressing no opinion on whether this is ok or not, but there has been a chain restaurant in the northeastern US catering mainly to families whose signature dessert was a "happy ending" sundae for decades.
And it appears that they do not use that phrase any more, but I found evidence it was eliminated in the last 3 years. I don't know why - it might have been a combination of changing sensibilities and their bankruptcy leading to reexamining things.
(2018)
"Get a free Happy Ending Sundae when you try any of our 5 new entrées!"
- friendlysrestaurants.com