Yes. I am a nurse on am medical floor in an acute care hospital, and anytime we have a diabetic with an infection, their blood sugar is higher. As the infection clears, his blood sugar will normalize.
If your FIL is a non-insulin dependent diabetic(ie-he takes pills or controls his blood sugar by watching his diet), he may actually have to take insulin while he's hospitalized to better control his sugar.
Generally, when we have a diabetic on my floor with an infection, we check his blood sugar 4 times per day (before breakfast, before lunch, before dinner, and at bedtime) via a fingerstick and have parameters for insulin coverage. For example, if I check someone 's blood sugar before breakfast and find that it is 210, I might be giving them 4 units of Regular insulin, which is a quick-acting insulin that will help bring that elevated sugar level down relatively soon. The parameters are usually set up by the physician Often, the person will also be on a slower released insulin(such as NPH or 70/30) which gets released more slowly into their system, in an effort to keep the blood sugar at a more even keel throughout the course of the day. Often, even if the person is on a slow release insulin, we will still do the 4 times daily fingersticks with Regular insulin coverage.
I got a little off the track there, but the bottom line is that any stress (infection, emotional stress, other physical illnesses) will affect blood sugar levels.
If I can help you with anything, feel free to PM me.
I am not sure how much say you have with regard to your FIL, but if he is not being seen by an Endocrinologist, it might not be a bad idea to request a consultation with one while he is hospitalized. They are the specialists in diabetic management.