@EnigmaGrey Ok, we went on co-op with the vehicle disabled, with no avail.
A few things to ask:
1. Is there a way to view your ping? That could be useful.
2. Since I know how to, I manually ported forwarded the ports needed by Project Zomboid (16261, 8766, 8767), would the conflict with anything
3. I was watching him in admin mode and made him invincible so he wouldn't die if he lagged. He reported that when the server started, they lag was noticeable, but wasn't hurting gameplay. The longer he stayed on the server however, the laggier he got, to a point where zombies were rubber-banding. At this point, he claimed that he was at the point where the game was unplayable.
If you need me to send anything, just let me know.