Ruin his life? Mateer did it entirely to himself. Posting the title "sports gambling" for a bet as a professional athlete - two payments, including one of a game in his own conference, on a Venmo account under his own name, no less; it's not rocket science to look that up, and I guarantee NFL teams do the exact same thing. - is stupid, and you don't do that as a professional athlete, even as a joke. People weren't nearly as sympathetic in other cases with young athletes who were around Mateer's age and did the exact same thing.
And here's the thing: his life isn't ruined yet. The maximum suspension he's facing is six games - if that. He'll have to answer questions to NFL scouts and GMs. He'll still likely go to the Combine, since what he did is not a criminal offense. He could possibly get drafted if he plays well enough.
My question is this: do you honestly want someone this stupid as your signal-caller for the next however long after Stafford? And do you think McVay would have any patience with someone this stupid?