Well honestly though - if they discussed the dinner and party beforehand and Lucy knew she couldn't go because it would make her late, \didn't she already know ahead of time that it would make her friends late too? By not pointing it out to them that it would be a problem to her for them to show up late, wasn't she implicitly saying that it was okay?
No, I don't think so. Instead it would seem her friend's would have a moment of awareness and think, "Oh, yeah, we'd probably miss a big chunk of the party if we go to dinner at that time. Maybe we should go to dinner earlier or skip the idea totally."
Well I kinda mean in terms of picking up on whether it is a come when you can or be here right at the start sort of party, as was being debated earlier. If they genuinely thought that it was a come when you can (and I've been to cocktail parties where people did show up an hour in because they didn't want to stay for the whole 3 hours that was set out in the invite and no one blinked), that would have been the time to correct them, but it wasn't done. They essentially told her their plan to show up an hour or so after the start time, and she in no way indicated that it wasn't appropriate for the type of party she was throwing. That is unless they changed the start time of their dinner after that conversation of course.
Not saying I would have done what they did, but there is some possible room before they get firmly into mean girl territory.
I'm not sure how you would correct them though.
Hostess Lucy: Hi Nya, I'm having a get together next Saturday at X restaurant. I've reserved the party room starting at 8 and we have it till the restaurant closes at 11.
Guest Mary: Sounds great. See you then.
Two days later.
Guest Tess: Lucy, Mary and I wanted to get a group together for dinner before your party. Do you want to join us?
Lucy: Oh, I'm afraid dinner would take too long for us to make it to the other restaurant on time. And I need to be there a little early too.
What is Lucy supposed to say? "Well, Tess if you have dinner at that time, you'll be late for my party." That comes across as a little pushy and maybe even like trying to control another adult.
To me, once Lucy indicates that dinner would conflict with her party, Tess/Mary should have realized it wasn't a good idea.