Your mother could follow a diet plan, but what's easier (I think) is for her to create her own. No matter what diet she follows, she should verify on her glucose meter that the food she's eating is, in fact, blood sugar friendly. Her own meal plan would also be more flexible and cater to her needs rather than the rigid constructions of someone else who doesn't know her.
Although this goes against common wisdom, carbohydrates - and not dietary fat - have a big impact on cholesterol levels. When a diet is both high in carbohydrates and fat, that tends to put on weight and raise cholesterol. When a diet is high in fat and low in carbohydrates, the opposite usually happens: cholesterol and weight go down.
The problem with the DASH diet is that its grain and fruit recommendations are VERY high. I don't know many diabetics who can eat that many carbohydrates without high blood sugar. I'm sure some people could lose weight on it, but it's not really the best diet for diabetes. Diabetes is about finding a diet that's right for multiple needs.
The only issue for your mom is that she's a vegetarian, so she'll have a more difficult time limiting her carbohydrates than a carnivore like me. In general, the food she should be limiting is grains, rice, corn, potatoes, oats, beans, sugar, and fruit. The food that tends to be better for blood sugar is meat (which you can exclude), eggs, cheese, nuts, seeds, and non-starchy vegetables (broccoli, cauliflower, asparagus, etc.).