I have been in a similar situation, and found it best to avert my eyes. Not to hijack the post, but here is my story. Mind you, this is all the same family.
1. We went out to dinner with a couple my DH is friends with. It was an all you can eat buffet that specializes in very delicious prime rib. (Its a small restaurant in a small town- not a big name place). I had been on Weight Watchers for a little while. My DH loves beef and has a fairly large appetite- but not as bad as this couple. The husband packed away 5 large pieces of prime rib- HUGE pieces as well as a bunch of other stuff, and no veggies at all. The wife at least had a salad before packing away about 3/4 of what her DH had. It was gross. I just kind of concentrated on my plate and tried not to look. On a side note- the husband died about 2 years later after having 3 heart attacks in one day. Anyone want to wonder why?
2. The wife (now widowed), her friend, DH and I went out to dinner. She didn't eat an extraordinary amount- but the forkfuls she shoveled in... eww. My mom taught me to take smaller bites, esp in public, and she practically had to unhinge her jaw to get the food in.
3. Birthday party for the friends son- same thing. Obnoxiously huge forkfulls of food in his mouth.
There really isn't a lot you can do except avert your eyes and pray for it to end.