That's really horrible. Poor thing! No wonder she was upset.
Don't let her be bullied like that. "Some people don't like it"? Yeah, well tough luck. I bet your daughter likes having diabetes a thousand times less. Diabetes is covered by the Disability Discrimination Act. I've found the mere mention of that seems to concentrate most people's minds. If your daughter feels unable to talk to her manager by herself maybe you or a friend could go with her for support? I'd have been nervous and upset too at her age.
My own daughter has epilepsy and she's dealt with discrimination at her first job. In her case it was more ignorance than malice. So it may well be worth your daughter enlightening them about the DDA and also diabetes itself. It could be the manager is just ignorant or thoughtless.
Please keep us updated. I hope all goes well.