I let my 8yo ride up front with me all the time. He was also 5' tall and 85 pounds. We also have 2 cars. One a SMALL car, with adjustable belts that would have the belt under his armpit at it's lowest setting so we keep it somewhere in the middle, and one that's a jeep wrangler (non adjustable belt) that had the belt 'perfect' from the time he was 7yo. In my mom's car at 5' tall he'd STILL be sliding around, because the seats are ginourmous. So he doesn't ride up front in that car.
I let him ride up front with me many times before that, starting as a toddler in a convertible car seat that popped his head up to even with MINE (and I'm 6' tall) because it was the safest thing to do or the only thing to do (equipment in the back seat, or no back seat). I just put his carseat up front which brought him to the appropriate height.
((Our state has a line in the law which states children need to be in the back "whenever practical". It's not practical if there is no back seat, or if there is dangerous equipment in the back seat, or if there is anything wrong with the seatbelts, or if you are transporting more children than there are backseats))
LOL... I was pulled over twice (seat related). Once by a rookie when I had no backseat (Poor guy was really embarrassed. "Sir? Where do you suggest I put him?" glancing back into the bed of the truck. His training officer didn't do a great job of concealing his laughter as coughs)... and once when I had my infant in a forward facing seat in the backseat. I had a note ("common sense exemption") pinned to the seat by the local seat inspector at the fire department. My son was too big to be rear facing, even though he was only 7 or 8 months old when I was pulled over. We had the biggest infant seat at the time, and he was over both the height and weight for it, so the fire department gave us the written pass to have him forward facing even though it was technically against the law.
For me, I've learned to follow 'common sense' ever since I had my baby in a seat that would have snapped his spine had we been in an accident. (oy!) In fact, I got chewed out by the fire department for having him in that seat, even though it was the largest on the market. "You need to listen to the SPIRT of the law, young woman, not the letter."
Okay. Duely received.