I'd say the ultimate fix is ACCEPT that some people have different tastes instead of trying to force your views on them.
Certainly true, though if it's specifically the fact that they're preaching how good Oblivion is because it lets you play after the main quest, the best thing is to let them see it isn't the only game with this trait. You can just let your friend play Morrowind, and if your friend won't play it, you can show that friend you playing it, best if you have lots of texture replacers and Morrowind Graphics Extender and such to dispel any "The graphics are too ugly." complaints, and them play through the main quest in front of that friend, to show that Bethesda has already been making games where you can continue playing after the mainquest before Oblivion, assuming that watching you play the game doesn't make your friend want to play it. If this doesn't change the friend's mind, then there's absolutely nothing wrong with it at all.
Oh it is, I know a guy who really likes Oblivion (and would love Morrowind), but his excuse was, "The graphics are too dated..."
That excuse is pretty true, though. With enough mods, Morrowind
can look alright compared to modern games, but this is only on the PC version, and only if you choose to download those mods. If you, for example, don't have a computer to run the game, or have a computer that can run Morrowind but it's not good enough to safely use Morrowind graphics extender and what not, the game still won't look good.
I would love to see him go look at Arena or DF graphics
I think that it can safely be said that, for anyone who would refuse to play Morrowind on the basis that the graphics are dated, one look at Daggerfall's graphics would be enough to drive that person away.