These are going to sound like dumb questions, but I'm going somewhere - when was the last time your friend changed his oil and what oil did he use?
I used Royal Purple in my jeep because I had always used it in my '71 Mustang with good results. A little while after that oil change, I noticed the exact same sound you're describing. I thought it was the exhaust manifold but couldn't find any cracks. I pulled the valve cover and nothing looked out of the ordinary.
I decided that before going crazy on a new manifold or taking it somewhere, I'd at least change the oil (and by this time, it getting to need changing anyway). I went with Valvoline Max Life (my jeep has 80k miles) and a Mobil 1 filter. I let the old oil drain out FOREVER too.
No kidding - 5 miles after changing the oil, I noticed the noise was noticeably reduced....and 30 miles in - it's gone.
Might wanna give it shot before your friend goes too crazy replacing stuff. $30 for something the jeep was going to need soon anyway was money well spent.