Please ask your mom to read the "wedding dinner" article; button is at the top of the forum page where you posted.
This is a pretty reasonable menu, here are a few important tips:
It takes 3 people manning the grill full time to do grilling for 120 people, and they can't be part of the wedding party, because they have to start cooking while you are still getting married.
For 120 people, self serve, you need 3 serving lines if everyone is going to sit down and eat. With 1 line, it takes 2 hours for everyone to serve themselves. Separate stations for the beverages (2 stations for this many people) and for the cake and desserts.
You put the appetizers (cheese/crackers
apart from the dinner food (I would do two stations the same) so that people can nibble while they wait for you and the wedding party to arrive.
For 120 people:
steak tips-1 pound raw for each 3 persons
chicken breasts- 1 small per person
hot dogs and hamburgers- these will hardly get eaten, and require buns and condiments. Suggest you change to grilled sausages,or sausage and peppers (which could be made ahead), instead- 1 pound meat for each 5 persons, plus mixed peppers and onions equal weight
2-3 pounds veggie burgers or tofu dogs for the non-meat-eaters
It really does take this much meat for a BBQ party meal.
potato salad- 40 pounds potatoes
pasta salad- 6-8 pounds dry pasta
fruit salad- suggest fruit trays, prettier, last better and leftovers are more useful, see the fruit tray page. The deluxe tray for 100 would be about right
grapes- suggest you incorporate onto the fruit trays
cheese/crackers- 16-20 pounds cheese, 8 pounds crackers
mini quiches-at least 2 per person- with 3 you would have some left over. Check you oven space and the time required to bake; these are not easy to do hot for large groups. You might consider sliced large quiches, which do not have to be hot, as an alternative
salsa/chips-1/4 cup per person plus 8 pounds chips
hummus/pita chips- 1/4 cup per person plus 6-8 pounds chips
Hope this helps.