How Clean Betta Tank?

She really needs a standard heater. Those pads only heat things to a degree or two above ambient (room) temperature and bettas need a minimum water temp of 25 deg with up to 28 being better.

Betta's tanks are cleaned no differently from other tanks. The only thing to watch is that with a small volume like this half the water is gone before you realise. The gravel should be siphoned clean with a standard siphon tube. The fish is usually left in the tank, though they are inquisitive fish and you have to keep an eye on them all the time or they can be caught while investigating the siphon tube. It is not good for a fish with long delicate fins to be netted out on a regular basis; this is only done when bettas are kept in very tiny, unfiltered tanks. And by very tiny, I mean a couple of litres as used by breeders.

Has your sister cycled the tank, or at least used media from the tank the endlers and gouramis were in? It is a common misconception that because bettas can breathe air, they don't need filters, or cycled tanks when there is a filter. They still get ammonia burns and nitrite poisoning. If not, she needs to monitor the ammonia and nitrite levels, and do water changes if she gets a reading above zero - you know all about that to explain it