I too would suggest keeping the refreshments light, but filling.

I can see myself (a reasonably experienced cyclist) getting bogged down or sick if I ate something like an egg sandwich and then got back on the bike! I can see inexperienced riders not thinking of that danger and just thinking about how HUNGRY they are (the second I step foot off the bike any morsel of food better watch out because I'm ravenous!)

I like the idea of bananas, and perhaps provide a few alternative fruit options as well. Sports drinks (gatorade, powerade, etc) are also a good idea, but have some regular waters as well (the sports drinks are high in sugars/high fructose corn syrup that the diabetics may not want).

To keep the larger spread idea that your friend wants to provide (good for her, she just needs more direction) here are some ideas:

Trail mix, or mixed nuts
Extra fruit
Salads (especially if this is a longer lunch stop)
Yogurt cups
Granola bars, powerbars, cliff bars (for there or on the road)
Consider Wraps instead of sandwiches (less bread, lighter and better for the diabetics)
Ice pops or sherbet (if keeping it cold is feasible, you can get this in sugar free for your diabetic friends too)

As someone who has lived in a family of diabetics and borderline hyperglycemic myself, some suggestions:

Diabetics can crash from too little sugar as well, especially when exerting themselves more than they are used to. So, too little sugar is just as bad as too much. Try to provide them with some sort of high carb bar (most granola bars are, as well as bars designed for sports) that they can keep on them. Bonus points if the high carb bar is chocolate coated (chocolate provides fast absorbing sugar to keep you from crashing immediately, carbs provide slow release sugar to keep you going). They make diabetic specific granola bars, but honestly the bars that are sitting next to them for half the price are the same thing

Your friends will know when their blood sugar is dropping (we can feel it, so if they are paying attention they'll get warning), but they may not know ahead of time how quickly their sugar may drop especially in the second half if they aren't used to cycling. IE: if they aren't used to it they may "be good" at lunch and not get enough carbs in them to sustain them with the increased activity. Hence why I think bars of some sort are a good idea. Easy to carry and will keep you out of a really nasty situation.