Every person's body and hormones will be a bit different but basically, they at first try to control it with diet. Many times by the last month, they may need insulin to help (just cause the hormones can make it difficult otherwise).
Proteins and veggies are typically best (best to have 2nd of these than other things). Starches will make her levels rise (things such as potatoes, breads, pastas, corn, some "beans"). Doesn't mean she can't have these things but it is about portion control. She shouldn't really have 2nds of these things.
What they told me when I was GD was anything in moderation is better than depriving myself and then eating a whole cake...better off to just eat a small slice and be done.
Basically she should follow a diabetic diet. One thing to note is that unlike the regular diabetic, they are looking to regulate her levels until the baby comes...they aren't looking at long term. What I mean is that I read that sherbert was better than ice cream for a diabetic so I thought great...I like orange sherbert better anyway. For a true diabetic ice cream is a concern due to fat content (weight and heart concerns since often times a diabetic has more than one risk factor) so sherbert is better but the sugar content is higher so for those with GD, ice cream is better (but just one scoop!).
The other thing to note is forget the food groups as you learned them as a kid. Dairy (cheese for example) isn't always dairy...it's also protein (for example, I was to have dairy and protein in a snack so I had milk and a cheese stick).