People only say that when they want their favorite PF to be the best.
Tim Duncan, in his prime, was a pure Power Forward. He was quick, agile, smart, and a beast on defense.
Ask your friend why he played so well with David Robinson, Nazr Muhammad, Rasho, etc if he were a center himself. He usually guarded the opposing team's PF as well.
However, in 2006 he had an injury that slowed him down a lot. Popovic tried to play him center once the playoffs came around, and even though Spurs lost the series he became inlove with the idea of having Duncan at center.
Which is why since, Pop has been inlove with Small-Ball and the front office only gets PF's for Duncan (Orberto, Bonner, McDyess, Blair, Diaw, etc.) Which is where your friend gets the "Tim is a C" idea, hell it's where anybody gets the idea.
I don't like it. For one, we haven't won a real championship since (07 belonged to Phoenix, tbh. Plus Cleveland and Utah were too easy). and even then we had Elson to pair up with him at times.
For second, Anytime Tim goes up against a strong center he doesn't do well (Haywood, Dampier, M.Gasol, Perkins) and that usually plays a factor every year.
Tim as a PF=3 Championships, 2 MVPs, and 3 Finals MVPs
Tim as a small-ball C= 1 "Championship".
By the way, at Power Forward Duncan had execllent competition. Kevin Garnett, Dirk Nowitzki, Chris Webber, Sheed Wallace, etc.
90's Was the decade of Centers. 00's Was the decade of PF's. 10's is looking like the decade of PG's.