OK, so you know this how?
You will advise him on doing what? Buying a new rearend on the prayer that you are right?
If he, ah hell, whatever.
Listen to this guy, it will only cost you $800 or so to see if he is correct!!!
If you ... Ah, forget it.......
How many belts would you like him to pay to have put on?
He did not go to napa and buy a belt. He did not put it on himself as he said I had a new belt put on.
Why would a all most new belt and a new belt be slipping?
Again it is not the belt.
Could be the key on the pulley that lets the pulley slip on the shaft going up hill ( have had it happen )
Could be it has done this for awhile is why someone gave it to him on a trade.
Could it be that only you know everything? No that's not it.
Could it be that many many many many people that have the 130 junk Deere have the same problem? Yes
Could it be that someone did not agree with you? Yes
Maybe he should just keep trying new belts until like magic it works again
(( cowboy up and get over it ))