Well your friend is wrong Gogeta cannot beat Vegito because potara produces the stronger fusion, so Vegito > Gogeta.
If you compare the same forms Vegito always wins, but if it is comparing two different forms then its a different story. So if it is UI Gogeta vs VB, Gogeta stomps no difficulty whatsoever. However if it is UI Gogeta vs UI Vegito then Vegito stomps.
Better = Superior = Stronger.
If Elder Kai said that Potara is the superior fusion that would mean that Vegito is stronger. If he wasn't, potara wouldn't the be superior fusion.
Quite an imaginative leap. While "stronger" does mean "better at strength", "better" doesn't mean squat. For all you know, he meant that because the fusion is permanent (not counting the retcon), it's better than a fusion whose lifespan depends on how much power you use (like the Metamoran fusion).
A fusion has many characteristics, one of which is power. Some people just decided to imagine that Elder Kai said "stronger", when, in fact, he said "better". Could be better because it doesn't require both fusees be at the exact same power level, could be better because it doesn't require both fusees be roughly the same shape and size, could be better because it doesn't require perfect coordination, and so on. Many things he could've meant, and only one of them is "stronger".