Hi I am just a parent with a child that has Diabetes for a Question like this you need to seek medical answers from a Doctor as this could be a lot of things from an allergy to a complication
I haven't heard of specific "tongue swelling" as an issue with either kidney disease or diabetes, but in general fluid retention is an issue with kidney disease. Tongue swelling can also be indicative of specific allergies. She should talk to her doctor about this symptom because it can be scary if her whole tongue swells enough to block proper swallowing.
Do encourage her to talk to her doc.