It's a shame how some parent's just don't get it. Where is the courtesy here? First of all, you invite her over, and I am taking this as she has been your friend for years? At one point or another, you must have told her that your daughter is allergic to ice cream. So now this gives your friend the right to allow her daughter to bring ice cream over the house? No!!! Especially, when everyone knows how children love ice cream, and now your daughter must be feeling so "left out" because she cannot have any ice cream due to her allergies.
You received some very good advice from AD members, and advice that you should follow through with. This friend of your's needs another reminder, that courtesy should be extended in your home, and respect is needed where your children are concerned, especially when food is taken over to your home and cannot be shared by your children. Where is the common sense here with this friend of your's?
Another thing, it seems like one of her daughter's who likes to be the bully to your daughter, is a copy cat role from her mom. Your friend needs to stay out of your children's business, and watch what she says in your home. Certain things that are said can hurt your children, and that's not acceptable in your home, and she needs to be told this, but, the way your friend behaved with your daughter, gave her daughter the right to treat your middle one the exact same way. Her daughter feeds off her own mother's attitude, which your friend needs to watch.
Yes, you definitely need to talk to her. This is upsetting your and stressing you out. Your family comes first as MsGiglz said, not your friends.
Go for it, get it out of your system and feel better that you did face your friend and tell her how you feel.
Then go have a drink!