IDGAF who's side you're on for supp singed
The answer and entire solution has already been said.
ADD THE GUY AND PLAY WITH HIM. He'll probably help you win more if you truly want to defend him BE HIS FRIEND, JOIN HIM. Stop being dam instagators on the issue, it isn't helping either side.
"If he gets banned, I quit" Ok so he's going to get banned and you didn't do shxt for him.
"Riot needs to know it's wrong" yet gave him the real solution just don't play with pubbies, play with people who know you. Yes it'll cut his playtime but at least he won't deal with bullies.
"Wtf, I've had several supports who don't build sightstone" those players don't do so 24/7. Not to mention after several times asked they'll alter their build or change roles.
"Lots of supports roam"- I don"t have specifics but not all adcs have the power to stay safe vs a 3-4 man turret dive or vs agressive lanes where roaming is strictly not an option due to poke or all-in. Yes a lot of other players would still roam but you do have to change playstyle for said cases. But I doubt this was the issue, hopefully.
"Refusing to comm with team"-Kind of on the fence while I don't think the paragraph is enough, I doubt he wouldn't drop occasional he's going to roam or play safe.
"He took farm from his own jungle"- I gotta say unless someone has evidence, a plat player wouldn't do this. I played ivern supp with smite sometimes, i would lane at a summ disadvantage before I would dare take a jungler's farm. I bet if anything he'd counter jungle the enemy and run away, but I doubt stealing his own jng camp was an issue.
"He would proxy"- This I'm scared of being a case, it works for top but in a duo lane too much people to reliably kill you. So I do think this is a lie, I havn't memorized his kda but it didn't show high death if I remember correctly.
"Let's all play supp singed"-first stop the stupidity none of us are even remotely good to get where he is with it and noone is throwing their challenger/master for it. Not to mention since noone good would perform using it you'll only make his situation worse and give more reason for him to be banned.
"But people played mf support"- yes they did she counters zyra hard, after people realized that's the only reason they stopped forcing her. As for worlds it enforced a counter-strategy which hasn't been done by off-meta niche picks in I would say years. Ofc noone would be banned for it.
There are many more things to be said, but overall again if you wish to defend him, do it right. The guy wants to play singed, he doesn't want to beat skt, he doesn't want to turn riot on it's head(would be hilarious if he did xD), he just wants to play singed support and climb.
Be his friend, play properly, defend him if he's in the right with the raging teammates not riot who's in a bind.
(I probably didn't address 25-50% of the issue but the main point is he has to play with real friends not the entitled pubbies. I also doubt that he isn't aiming to win or he wouldn't be plat so he def won't fk you over if you practice together)
And before anyone ask, I'm not on the defense. A person who's not in agreement that the singed player is fully right, can never be. Even if I tried being on his side, I'd be a hypocrite because I wouldn't play with him. So please I beg of you join him if you truly are on his side do it for him. Don't do it for riot or if you secretly hate his playstyle you'll only hurt him further.
Not saying you can't help him polish support singed futher, but don't do it because of a bad game. You'll only make it sound like it was his fault.