Is she a long-standing diabetic? And is this her usual practice? Is it Lantus that she takes every day at the same time, but on sundays she's able to sleep in, so she takes it and goes back to bed? If any or all of these are true, that seems totally reasonable. If it's Lantus or something similar, I wouldn't give it a second thought, especially if she's experienced with insulin. A lot of people take it either right before bed or first thing in the morning.
It's interesting to me that you keep tabs on what she's doing. I was dx'd about four years after my husband and I started living together and he's been watching me deal with it for 20 years now and I don't think he has any idea what I'm doing or when. I made him learn how to test my blood sugar for me during my first pregnancy in case I needed help while I was in labor or something, and that, nine years ago, was the last time he gave it a thought. He does wake me up in the middle of the night if I get too fidgety, though, which is probably a godsend.
Anyway, it's great to be supportive, but be careful. Years ago when it was all new to me I felt a little put out that he was so uninterested, but now I think I would be irritated if he tried to second guess me.