Does train wreck entail poor timing, poor technique, poor note choices, not syncing with the drummer? All of the above? If the player insists on playing live when unqualified, focus on whatever stage readiness skills they need. There is a music school where I live where they put equally untalented players together in a group to learn together how to jam. Then they have showcases for family and friends. It’s maybe faster to learn by playing with people at a higher skill level than yourself but it keeps trainwreckers together and away from the rest of us.