I've towed multiple fivers with GM half tons, older trucks with less capability but also lighter fifth wheels (even fewer on the market today). Towed with the 3.42 ratio, many years and lots of miles but much happier with a 3.73 ratio. The biggest challenge is finding a fiver and other cargo/passengers that won't exceed the truck's GVWR, or at a minimum the rear GAWR. No point in stating my older numbers for pin weight, GAWR, etc. That will need to be determined for his truck, loading and potential fiver. I will say that my last half ton also had an aux 40 gal fuel tank in the bed yet I was still under the rear GAWR.
All of my tow vehicle have be 6.5ft bed GM trucks. My first setup (with the 3.42 ratio) was so long ago, it was before slider hitches were even on the market. All I had was an extended pin box. And the fiver didn't have a curved front cap like today's units. Same setup with the second truck, total of 14 yrs without a sliding hitch, three different fivers and never damaged the fiver or truck cab. If the truck in question is a 5.5ft bed and the fiver has a curved front cap, then he won't be any worse off than my older setup. However, a manual slider isn't that expensive or heavy compared to a fixed hitch and used ones are readily available. Have one in my current truck that just isn't used.
Anyone that says a hitch needs to be mounted with the king pin 6" in front of the axle doesn't need to touch the truck.
Finally, I do agree with the husband's view regarding TT vs fiver. While challenging to match a fiver to a half ton truck, I would rather tow a fiver at 100% of the truck's ratings than a TT at 80% of the ratings. Just more stable and less driver fatigue. Even at the end of a long day, I still backed into a site w/o hitting the back of the cab going on 24 yrs.