Just for the record, you can start the diet +/- insulin without hospitalization and then follow up in a few weeks. When I've started diabetic cats on insulin, I typically do not keep them in the hospital for a number of reasons: They stress out too easily, and that will usually mean they may not eat very well (or at all), and stress alone will make a cat's blood sugar spike up for a short time - so if I keep checking blood sugars on an already stressed-out cat, they may still be high regardless of starting insulin. Or could be affected by getting insulin without eating well. I would show the owner how to the do the injections, have them call me if any issues, have them start Kitty at home, and then go from there.
There is a blood test that can be done that checks for levels of an enzyme called fructosamine - it can be used as a measure of blood glucose (sugar) control over the previous 2 weeks. So if there is good blood glucose control, the fructosamine level will reflect that by being low - if the fructosamine level is high, then control for the previous 2 to 3 weeks has been poor. Whatever momentary spike in blood glucose that a cat may experience from stress (i.e. trip to the vet, blood draw, etc.) will NOT affect the fructosamine reading. So in my book is a much more preferable method of monitoring diabetes control in a cat - rather than having them have to stay in the hospital and get poked every hour or 2 for a blood sugar reading, they come in for one quick blood draw and that's it.
There are also some insulins that only have to be given once a day rather than twice. One is Lantus, which is more pricey compared to some others but I've gotten good results with it. You might want to ask your vet about it, and about checking a fructosamine level in a few weeks, and see what she thinks.