Can sol ring only be used in commander? Friends of mine are saying it is but I don't think so.
Nope, not at all. It's just most commonly played in commander. I don't remember if it's legal in Legacy, but it's definitely not legal in Modern or Standard. Casual is fine though. You're correct.
It's actually banned in Legacy, restricted in vintage. Puts it on the same level as the p9.
So if you do run it in tabletop I'd advise not to run a full playset due to power play.
{Sol Ring} is a super powerful card. It's an absolutely insane card in Affinity which is a large part of the reason it's banned/restricted.