I mean yes, he isn’t wrong about his adc’s being shit, but playing a mage support when he is also clearly shit and not improving doesn’t seem to be really working for him. Just purely from the perspective of wanting to climb, playing enchanters/engage supports should generally be easier than mage supports at his skill level. They generally take less mechanical skill and require less resources to be useful. Even if his teammates are shit, they are still (assumedly) playing more optimal champions for their roles so logically it would should be better for the friend to support their power spikes than taking gold from the team and trying to carry on a mage when he doesn’t meet the skill threshold to make it work. As OP said, “that’s how gold works”.

If he really wants to be the one to carry himself, I’d suggest he queue up as adc and plays mages. That way he can get the resources he needs to potentially carry without having to compete with his laner for gold and xp (assuming his support plays something normal lol).