No kidding. How sad and boring it must be to live in a world of just documentaries and the non-fiction section of the library.
I would believe my wife in a heartbeat. She is very level headed, brutally honest and not prone to exaggeration. Plus the fact that she hates sci-fi and fantasy would mean that it definitely happened as she said. Plus our friends are pretty much of the same mind when it comes to sci-fi/fantasy.
I like fiction just fine, even some fantasy (as long as it doesn’t take itself too seriously), but that’s not the same as trying to hypothesize it as reality. A universe where the supernatural is possible is a universe where I wouldn’t be me and my wife wouldn’t be my wife, so it’s impossible to say anything meaningful about that hypothetical.
OK, let’s tweak the OP to avoid the fantasy world scenario and try something theoretically possible, although extremely unlikely.
Theoretical physicists have hypothesized about parallel universes. What if they are correct and they do exist? Under particular conditions a connection could be made between a different universe and our own. Again, keep in mind that the odds against this are infinitesimally small, but theoretically it is a possibility. If your spouse and friends were sucked through this rift (at the same time that beings from that universe came in to ours and caused havoc) and managed to fight their way back, would you believe them?
No, the odds would be astronomically more likely that they were lying, even if I thought they were honest people (which my wife definitely is). The possibilities of some kind of mental illness/hallucination, drug induced delusion or elaborate deception by others, as far fetched as they may be, would still be much more likely than the possibility that they actually slipped into a parallel universe and then slipped back. Occam slices this one pretty thin.
Even if you witnessed creatures that appeared to be of non-terrestrial origin or phenomena that appeared to defy standard explanation? Keep in mind that the OP stated
I would not witness any creatures of non-terrestrial origin. It couldn’t happen. I reject that premise as impossible. A universe where those things could occur is not this universe, therefore I wouldn’t exist in it.
Belief wouldn’t be that hard; while all of our friends would certainly have the background to make up such a story, none of them would present such a thing as real. The eyewitness errors would be more likely to confirm for me, especially as I’ve got a good idea what things they’d disagree with each other about.
It’d bother me that it was her and not me because fantasy quests are terrible things to experince–I’d rather that she worry for me for a week than spend the remainder of her life carrying wounds from saving the world.
I’d believe my husband – he doesn’t and can’t make stuff up.
But I’d be pissed off that he didn’t take me – that’s supposed to be my kind of thing. 
I’m actually mostly on board with Dio. The scenario in the OP would cause me to doubt my own sanity.
50 years ago the “earmouse” would have been impossible. Someone 100 years ago would have thought the Space Shuttle was impossible. A human being with mechanical devices replacing limbs would have seemed outrageous 200 years ago.
Test-tube babies, DARPAs self-driving cars project, nano-technology were all impossible. We’ve seen a dead woman’s face sewn onto a living persons head! Why is it impossible to conceive that if life is capable of taking hold on Earth that it couldn’t happen elsewhere? Scientists regularly talk of multiple dimensions beyond the 4 we can perceive. Who can say there aren’t more?
There are more things in heaven and earth, Dio, than are dreamt of in your philosophy.
No there aren’t. Hamlet was full of shit.
There is a difference between something being technologically impossible, and actually being physically impossible. Flying through the air was once a technological impossibility, but not a theoretically or physically impossible one. Flying faster than the speed of light is physically impossible, regardless of any conceivable technology. So is magic. So is time travel. So is travelling into alternate universes. These are not even theoretically possible, and technology can never make them so.
I might be able to believe that they believed it. I would have to leave it as a big unknown. Maybe that was real, maybe they all had a group delusion. I would accept it as possible, and then wonder for the rest of my life what really happened.
It’s because of being left out of an epic, life-altering experience that you can’t begin to imagine or share. Partners are teams, we want to be included especially in the tough stuff.
We’ve got three kids. Mordor would be a cakewalk after that.
Are you saying your children are orcs?
Seriously though, like what’s been posted already, if the crazy stuff happened ala the OP, then I’d believe them. If the events were more mundane… I’d only believe them after a long while.
Heh. I’d believe it. Me, I SUCK at so much as camping, let alone a “quest.” I’d eventually probably believe it and then tell them thanks for not bringing me along. Then I’d be all, “But you brought me home a crown, right?”
A week?
Not in a relationship of that nature, but I lean toward not really believing that’s actually happened but saying, “whatever,” & then coming over time to suppose it probably did.
That said, maybe she just went off on a drunken binge & is making excuses.
Or maybe she’s the cause of the horror & someone else stopped her! Considering she disappeared before all this started, that fits the timeline as well–no, better! Unnerved, I come to believe this scenario; I eventually strangle her in her sleep.
…Yeah, I’m never getting married.
ETA: No, really, seriously. You don’t know my friends. The closest people to me are the sort who would absolutely just make something like this up just for giggles. I never know when they’re telling the truth. Ever. My father was the same way. To this day, I’m not really sure about some things…
I would be upset that I was not invited.
If Dio told me it happened, I’d believe him…