Requesting the mentor regarding the PhD research topic selection
It is a mistake for most students (Ramanujan excepted) to go against his or her advisor. I'm sure that the advisors complete thoughts are more complex than you present here, but probably involve the risk of taking on a big project as a student and ending up with nothing - no results and no degree. Some problems are hard and take years-decades-centuries to resolve. 

The advisor is giving good advice and following it will put him or her on the side of the student. 

If "your friend" wants to take on the big project there is no problem, but will be working from a stronger position by finishing the doctorate first. 

Take the advice. It is the faster path to success.