I own a 1998 3930 4WD and I use it primarily to clip hillside pastures in Middle TN. I have found it to be a very tough tractor. It is easy to operate and service.
I know a guy that had a 3930 or a 4630 I can't remember. It is really under powered. He has had this problem with it since it was new.
Has he had it looked over? I have ran around 10 different 3930's and 4 3910's and two 4630's. All of them were great tractors with plenty of power for their tasks (pulling medium duty 5' Bush Hog rotary cutters), grape trailers with 3+ tons of grapes through mud and the 4630's were both loaders used to load 1 ton boxes of grapes onto and off of trailers. Only one 4630 was fwa, the rest of the tractors were 2wd. The only real issues was pulling a 5 ton lime spreader in a tilled field as the narrow vineyard models would sink in the soft spots. I had to hook the TN65 with 12' disc still attached to them to pull them through.
I really like the old Ford 10 and 30 series. They are very well respected and can take a ton of abuse. So I would have to think either your friends tractor has an issue with the motor or he is trying to work an implement that is too big for it.