What, could my friend have diabetes?

Lately my best friend has been very sick. She's been sick for almost two weeks now and it was kind of on and off. She started to feel better then she'd get sick again. Yesterday me, her, and my mom went to the mall since it seemed like she was feeling good enough to go. While there, my mom noticed how she was squinting a lot and my friend has said that she needed glasses lately and her vision hasn't been so good. She says that even when she sits in the front of the class at school, she still can't see the board. And she wouldn't eat at all. She wasn't hungry all she had was drinks, mostly water. She had brought a water bottle with her. My mom has been very worried about her and now I'm starting to get worried. My mom was talking with my stepdad and he is convinced that she probably has diabetes from those symptoms. Diabetes runs in her family. Her sister has it and as far as I know so does her mom and uncle. She is a little bit overweight and is sort of mistreated and neglected by her family. She hasn't been to the doctor for a while and her mom won't take her to the eye doctor to get her eyes checked. I mean her vision wasn't all that bad but all of a sudden it started getting worse. I have bad eyes too but that's mostly because I'm on the computer a lot. My eyes didn't start getting worse until a while so we find it weird that it's this sudden. And her mom doesn't even pay attention to how sick she is. Sometimes she feels so sick she even sits out during PE. My stepdad now suggested for me to mention to her to maybe go the nurse at her school and get her blood checked just in case because I know that diabetes can be very dangerous if left untreated and I don't want to take the chance of her getting worse and dying possibly. She's my only friend. So my question is, could she have diabetes and does she have the symptoms for it?