Some shocking levels of ignorance around diabetes on this thread.
This comment was spectacularly ignorant and very hurtful (to me, as the mother of a type 1 diabetic boy): "Why would diabetes make someone bite his partner's arm, she isn't made of sugar."
OP, lots of people on this thread have also confirmed that yes, his glucose may be changing his behaviour. High and low glucose levels can have profound impacts on behaviour, and once you reach a certain point you are out of control (and need help/intervention).
He has been to his GP; if he was genuinely abusive he wouldn't have bothered. It sounds like he has been fitted with a continuous glucose monitor: that should send alarms when he gets highs and lows, helping him address them. All hypos (lows) need immediate attention/treatment. If he doesn't have the alarms on his sensor he needs to go back to his GP and request one that does: my son has a Freestyle Libre 2, which connects to an app on his phone and mine. We both get alerts, which your friend's partner may find helpful. He should also be able to access more support: we have an adult diabetes centre near us, for example. Or try Diabetes UK for advice.
As to the injury causing the spikes: yes this is plausible. According to our diabetic nurse, around 50 different things can affect your blood glucose (making them go high or low), but as a diabetic you are only in control of 3 (diet, insulin, exercise). So when something happens to you that is out of the ordinary it makes it hard to control. My son had a MILD cold a few weeks ago, for example, and had massive lows and highs - nothing much we could do about it apart from ride it out.
Please all those who are being nasty about diabetics: can you stop and think before you type, please? Or read up on the condition first? It's pretty devastating to read some of the things posted here when my little boy has it - will he end up on the receiving end of abuse like yours, just because he has a condition he didn't ask for, and has to live with for the rest of his life?
OP, I hope your friend and her partner get the support they need.