Gently push FIL to properly care for Betta?

[MEWaters]

my wife and I were visiting in laws over the weekend
they've had a betta bowl for supposedly 2 months (looks like a half gal bowl) no filter or heat
clealry has a fungal infection, white fuzz near tail fin and on cervical aspect of neck
watched him do WC and only 'waterfalls' tap water into the bowl - cold tap from N ME
he's the type that stops eye contact if you engage in conversation or have any opinions (he's a chem engineer), and he has no room for outside knowledge or input.
Was simply thinking of a dechlorinator as a gift and mention to add with every waterfall and leave it be.. any other inputs besides mentioning nitrogen cycle

 

[Fish0n]

You could give him a large peace lily or pothos ready to float in the bowl. It looks pretty and could help stabilize between his water changes. Just be sure he doesn't get the impression it reduces the amount of water changes needed.
I gave a plant to a coworker with a betta in a bowl and she loved how pretty it looked and the water seemed clearer between her waterchanges

 

[MEWaters]

he has a plant that almost looks like a red ludwigia, but I know it's not - and it's melting like crazy on them.. sadly, I don't think more plants are the answer.
Unless I add plants with the water trx?

 

[juniperlea]

I'm assuming these are your wife's parents. Perhaps she can nudge them in the right direction. If he can't look you in the eye, I wouldn't......... Or.... for the next holiday or birthday, present them with a fully cycled 10 gallon kit, including plants and a few tank mates...... And, you could put food, a siphon etc in their xmas stocking (if xmas counts).

 

[Salem]

my brother is the same way- the only way I'm ever able to actually give him information on things I have to be really passive about it. for example- "oh wow did you know bettas actually live in rice paddies that go on for miles and miles? turns out they don't actually live in puddles I guess" or " so and so got [insert pet] and doesn't even know how to take care of it! I thought it was common sense to research what a pet needs before buying it but I guess not"

another route is to not try to actually talk about it and just gift it. you could perhaps get one of those 2.5 aquarium kits they sell at like petsmart? most come with a little filter and they usually sell tiny heaters for them too. maybe throw in a pamphlet about basic betta care? if he confronts you about it just be like "I dunno I just asked the store employee what I should get someone with a betta"

 

[Fish0n]

he has a plant that almost looks like a red ludwigia, but I know it's not - and it's melting like crazy on them.. sadly, I don't think more plants are the answer.
Unless I add plants with the water trx?

So I find that the peace lily and pothos are much better for bowl like tanks as they only have the roots in the water and grow quickly they are much more efficient and successful. The plant melt would also contribute to bad water quality whereas a healthy plant would have the opposite effect.