First, "KYS" is NEVER considered a joke.
It's something which is included in the zero-tolerance box for a reason.
There are real people on the receiving end of such comments; they have real lives.
Is there anyway to contact a Rioter for more clarity?
Your friend needs to contact them through a support ticket directly.
Player support will not disclose the info about his/her account to you.
Tell your friend to ask in his/her ticket to be escalated to a player behaviour specialist to talk 1:1 about their behaviour. The first response will most likely be "we've received your ticket" the second will likely be "we'll redirect you" and the third he/she should be 1:1 with a player behaviour specialist.
Note: I don't think the ban is undeserved due to the contents of the chat, but that's how to reach Riot to talk about it.