Yesterday I want to Petsmart while I was wiating for my aunt to finish up at Costco. There was a lady and her daughter by the bettas talking to one of the workers.
From what I gathered, the lady was keeping the betta in a small fishbowl by the window, unheated, doing 100% a week, and it had died.
Well, the employee said that she seemed to be doing everything well, except that when she changes the water she should only do a 50% change. She said "the water may be icky for us, but if you do 100% changes you'll be getting rid of the good bacteria in the bowl". She also said that he might be getting chilly in front of the window, no mention of direct sunlight.
But at least she had the sense to tell her that bettas are tropical fish that need warm water. She said 78 degrees is optimal.