Should I get a heater?

Actually, a lot of people do not keep their betta with heaters. All you have to keep in mind, bettas are TROPICAL fish. Therefore warmer water (78-82F) is their ideal. Warmer climates and warm houses can be sufficient for many bettas and I have seen bettas kept in un-heated tanks around 55-60F.
Bettas are truly a versatile species but mimicking their warmer temperatures is definetely what you want to aim for!
Heaters are cheap nowadays. You can get 10w-100w heater for your 2.5g

they will definitely survive without one but from my experiences they seem happier with them. i love my fishies enough i don't mind spending $10 on a heater for them. my cousins fish for example. she had a betta in a 2.5g as well. her room is really cold and the fish always acted lethargic and sick. so i got her betta a heater for christmas and it perked up right away. it would flare and its fins were no longer clamped. it was a happy little camper. another member on here once posted about how her cat unhooked the heater and she didn't realize it and when she turned it on the fish wouldn't leave the heater.

its up to you whether you get one or not but i'm sure bella would be very happy with one