My friend was diagnosed T1 when aged 40 so it can happen.
My son has T1 and I don't cook any differently for him.
As above, she needs to get some training on carbohydrate counting, as the carb value of the food relates to the amount of insulin she needs to have. Other variables will affect her blood sugar levels, illness usually makes them climb and exercise can make them plummet for some time afterwards (but exercise is GOOD!).
If she's on insulin with meals a good starter guideline is to test her bloods before, then two hours after the meal. If the levels are the same, she's got the dose right for that meal. If the level after is higher, she needs a bit more insulin, if the level is lower she needs a bit less.
The carbohydrate value of the meal will affect how much insulin she needs. Pasta, rice, chips, etc are high in carb. Eggs, veg etc are low. We use a collins calorie counting book which lists carb values. After a couple of weeks it's almost second nature!Leeds Uni have a great website, look here