Water parameter's and temperature are too unstable for long term health of the fish in anything less than five gallon's per betta.
These are some of the most abused fish in the hobby, due in large part to those misguided folk's who attempt keeping these fish in small vases,bowl's,etc.
They are tropical fish that appreciate/need, a heated filtered tank ,just like any other tropical.
Forum's are littered with post's regarding sick betta's and all manner of remedies to effect a cure, when simple prevention is easily obtained by providing the fish with basic care that other fishes also demand.
Forums are also littered with fok's who sometimes have a long RIP list of fishes that were lost largely due to poor enviornment/ignorance.
I have a sister who has killed,replaced more of these fish than fishes I have lost in last ten year's.
She keep's them in bowl's ,and changes water when it suit's her.
The fish struggle with fluctuating temperatures and swim in their own feces until they can no more.
they don't get sick overnight,,they slowly(painfully slowly),,become sicker and sicker, and then she want's info from me as to what med can she use that will bring immediate relief.
My answer has not changed and neither has her bull headedness.