In terms of 'live' food Bettas are carnivorous and love meats but usually stick to baby shrimp or brine shrimp or specifically good for them and their stomach sizes are bloodworms. Each Betta is different, so test fruit flies for your fish and if they show any signs of difference; extra flaring, weird coloration on scales, bump where the stomach area is or diffrence in the consistancy, quantity, frequency and coloration of their feces, then fruit flies aren't for your fish!! And again use the fruit flies like you would with the shrimp and worms, 2-3 times a week as a treat for your fish NOT an everyday diet!!
Love and best of luck,
Simi and Mali <3