She doesn't necessarily need to have snacks before bedtime. Given she is a small person it is quite critical that she follows a low carb way of eating to help reduce her insulin and stabilize her blood sugar (this is different from the conventional approach which says you can eat what you want and cover it with insulin.)
Please review and study "Bernstein Diabetes University" on U=tube. You will find lots of information.
Type 1? Are you sure it's not type 2?
Like Super_sally888 says, you do not have to have snacks before bedtime.
If her sugar levels drop low before she goes to bed at night, then that would be the time to have a snack, otherwise if she is not hungry, she can miss that out.
As your mum has only recently been diagnosed, her medication will need to be monitored and adjusted to suit her. Her doctor should have given her a chart and instructions when to monitor her blood sugars to see how she is getting on. Tell your mum to ask her doctor for a referral a dietician who specialises in diabetes. There is lots of information on the web too.
The types of foods to avoid are all sugary and processed foods. High fibre foods, nuts, seeds, meats are slow release. Cut down on saturated fats as this can spike up sugar levels. Doing some form of exercise will help to burn the energy.