Over 200k on my 5.9 Cummings and no real repairs to speak of. An alternator last week ($126 my cost)and replaced ball joints. My nephew is on his 2nd 6.7 Cummings truck and has had almost no problems. He did have the DPF go out at 110k on his first one and did a delete on it, which greatly increased his fuel mileage and performance.
I don't buy into the whole "fuel costs more" argument. Does it? I pay a bit more up front but get way better mileage than a gasser - loaded and unloaded. Seems like a wash until I pull my boat with my gas truck and only get 12-13mpg or less.(depends on what lake I'm going to)
If I was purchasing to pull a travel trailer, there is no way I would buy a gas truck.
Some simple facts. They do not make 5.9 cummins or 7.3 powerstrokes, or emission free 6.6 duramaxes any more and haven't for some time, unless he is looking for a well used high mileage truck its not even worth discussing.
The 6.7 you nephew has... the DPF fix is a several thousand dollar fix, deleting it isn't much less unless you do it yourself which most people do not have the skills to do. Also plain and simple deleting one is against the law and you commit a crime selling a deleted vehicle and dealers will not buy them. Just all around a short sighted fix.
And in all honesty the fuel mileage is not night and day. You don't get 20 mpg driving a diesel however you want to, if you drive them both similarly they will both surprise you, I know a lot of people who only get 13-14 mpg out of a diesel empty.