I think The Scenario is probably more common than we're willing to admit. Without a pre-discussed plan, it's difficult for lower-time guy to second-guess a higher-time guy, especially when he's the owner of the ship and has "done this before" (how many times have we heard [i]that![i]).
In this particular case, the flight is well and truly screwed up if indeed it is IMC at 100' agl. Bad, bad place to be, "familiar with the area" or not. That's no excuse. It's not good practice, it's not safe, and it's not a situation I ever want to find myself in.
So helmet fire has it right: The NTPIC (not the pilot in command) must assert himself loudly and diplomatically. "Look man, I am REALLY uncomfortable here. Land this thing NOW and just let me out. You can go ahead and kill yourself if you like, but not with me in here."
The PIC will probably roll his eyes and call you a wussie. But he'll probably relent. And so you get out. And maybe the guy takes off again, and maybe he makes it to his destination while you have to scrounge up a ride. Who's the dummy now? (Hint: It ain't you.)
On the other hand, we've all found ourselves in bad weather. I'm not to proud to admit that there have been times when I've been sitting there sweating, heart pounding and saying (not out loud though), "Dear God, please get me out of this and I promise that I'll *NEVER* do it again! In fact, if you get me out of this, I'll start going to church on Sunday and stop touching myself so much...well, I'll cut back a little," all the while maintaining that calm, cool, kind of bored, ops normal demeanor to my pax.
The problem with aviation is that often there are no clear-cut responses. One man's "I can make it" is another man's declaration of emergency, which is tough when they're both in the same cockpit. Sometimes the former guy crashes; sometimes he makes it through okay, which only reinforces his willingness to try it again next time.
At the end of the day, SASless is right- things need to be sorted-out prior to take-off. But that's no guarantee, because even with such an agreement, the go/no-go line can become arguable...let's say "discussable." It is why I do not like flying with other pilots in single-pilot a/c - even in good weather.