Reading other replies, I noticed that no one suggested turning the attention to the guests. And by that, I mean making them feel that they need to leave because they have other things to do. The question ‘how do you get back?’ can be interpreted as a general one. That question also can be thrown around to see if some guests can offer a lift to others if they are heading in the same direction. But what about making the guests’ plans a priority?
If guests have pets, it’s never a good idea to leave them for too long. Asking questions about – how the pets are doing, what they are doing when they are left alone, and if they are okay being left – can switch attention to what is going on at home and urge the guests to go and check on their cats, dogs, etc. Also, they need to be fed, so pet owners either have limited time to go out (but sometimes forget about it) or need to hire/ask someone to look for their pets. In both scenarios the sooner the guests come home the better.
Almost the same if the guests have small children that are not visiting your friend. Directing attention to their kids could make the parents think about how they are doing at that very moment and make them want to go home. If small children aren’t good with their parents leaving for too long, it’s easier to prevent a disaster than to deal with it for at least a day later. Your friend could make those parents remember that there’s someone waiting for them to come home.
If your friend has a dog that needs to be walked at a certain time, that’s a good excuse to make people leave. They need to understand that your friend has responsibilities outside these (family) gatherings.
Those were simple general examples of topics. I assume your friend knows specific occasions that can make every individual think about going home to do their important thing. It can be: getting some rest before the beginning of a hard week, getting ready for a vacation, doing chores, going to the store before it closes, going to watch a primera or a live event that can’t be postponed, getting on a road before it gets too dark or crowdy, etc.
It's totally another thing with pre-set boundaries or expectations. Sometimes it’s even easier with family members because well, they are family. They probably love and support you. Especially if they want to spend so much time together. It’s not blunt when you’re sharing your feelings. But I would suggest finding a few family members who would like the idea of shorter visits, so they could jump on it in time to support any changes.
I would probably phrase it somehow like that during one of the meetings, “I love you all and I’m glad that all of you could come to visit me. I’m looking forward to the next (family) gathering a lot and I really hope it happens soon, not just on another holiday xd. I hope you get back home quickly and safely because I cannot imagine myself surviving a road trip right now. No matter how much I love these, I feel totally spent. So please, if you have plans or feel like leaving feel free to do that. And to not shatter the mood, I wanted to suggest meeting more frequently but make it shorter. I know we all have schedules and busy lives so maybe escaping from it all for a couple of hours once in 2 weeks would be better than spending a whole day once a month or during holidays. Like friendly catch-ups, we might as well go to some places together. Picnics, for example? Or bowling.” Depending on people’s reactions your friend can organize the next invitation. There always will be people who won’t like the change, but I’m sure someone doesn’t like how it goes right now either (not to mention your friend herself). I hope she realizes that she doesn’t need to be present during those gatherings all the time. Everyone needs moments for themselves, so going to a quiet room/place during the meeting is important to remain sane. People can easily entertain themselves. Provide them a bit of food and drinks and they are set.
I hope it was at least a bit hopeful. Good luck to your friend! If I think about some other suggestions, I will add them.