Under those circumstances, I'd have put some dessert on a paper plate and not thought another thing about it.
I had one girlfriend on the Carbohydrate Addict's diet and she had to eat her one meal with carbs in an hour's period of time. So at first she'd ask if she could go ahead and partake of dessert right after the meal, which bothered me not one whit since my DH likes to eat dessert right after the meal (too soon for me).
After a couple of times, it was just expected (we traded hospitality a lot).
But if this was typical behavior, this 'eat-n-run', I'd be put out.
Back when we lived in NC, we were good friends with two couples that each had a 16-year-old boy that were best friends with each other. I started to notice whenever we'd get together that the two besties would show up late, shovel down their food and take off. The first time I really noticed it was when we entertained at my house but they did it at everyone's.
So the second time we had them over, when they started putting on their jackets to leave (they did, at least, take their plates to the sink) I pointedly asked, "Where are you going? We invited you to spend the evening with us. You are just going to eat-n-run?"
Cue stammering by them and their parents, but at get-togethers beyond that, the boys made more of an effort to hang out before taking off.
It made me feel like a short-order cook.
As far as the OP, I usually go ahead and have the desserts out at a buffet for those that like to satisfy their sweet tooth immediately after eating - so this wouldn't have been an issue anyway.
I do think that it would've been better for Bill & Reba to make their apologies at having to leave early and, likely, the host would've offered to send them home with some dessert (at least, I would have and so would all of my friends).