Filtration would be a hob of some sort. I wish we had a fresh spot too.
I mean, apply what you know from saltwater here. Algae grows when conditions are good, and algae really doesn’t care ;-)
This is freshwater, no corals or anything, so if you could strip the water of all nitrates and/or phosphates you should kill off the algae with no issues for the Betta.
Maybe look into Phosguard or similar:
https://www.petsmart.com/fish/filters-and-pumps/filter-media/seachem-phosguardandtrade-70898.html
At 3 gallons, water changes would help, but with tap water you’re always adding back nitrates and phosphates. Possibly your friend could do like half tap water and half distilled water? Definitely don’t do all distilled water.
I’d also try to limit lighting. A Betta doesn’t need a light at all, so maybe turn the light on only when they are around to observe and enjoy the tank (meaning, don’t leave the light on all day while at work, for example).
If strong, natural sunlight is hitting the tank from a nearby window, that can cause this growth, so keep that in mind. And keep in mind, the sunlight changes by season, and if, for example, your friend is at work all day, they might not know the the tank is getting blasted by sunlight at some point during the day.
Oh man….. I’m I going to get kicked off of R2R for talking about this??? Shhhhh, don’t tell anyone we are talking about a freshwater Betta ;-)
(#)Don’tTellRevHT