The majority of people on this site are the type of guys who are very careful about maintenance of their equipment. That is a very good thing.
But there is another breed of equipment owner who doesn't give a rat's *** about maintenance. They feel like it should run, until it doesn't. Your friend could be one of those people.
If you told him you're going to change the oil and he's OK with it, do it. It might be the last oil change the machine sees! In all likelihood, if something unrelated goes wrong with the machine, it will be your fault either way.