[tigerwoman]

guestimating 1/4# fruit per guest no more possibly less
off the top of my head I would go with
2 cases of strawberries
2 watermelons
3 cantaloupes
2 honeydew

but really I would cut down on the melons and substitute grapes.
Just make sure to cut them into small clusters
much less work, they don't get that glazed over look and the color variations are nice.

another nice fruit garnish is to use orange slices or mini kumquats

you should find out how much cheese the mom is bringing.