Overall, my experience has just been not to try too hard to guide new players experience. Sure, don't pick a chapel/mountebank layout and don't just hammer the power strats yourself, but much of the fun of Dominion is figuring stuff out for yourself and trying the cards out. If they're not into trying the different cards and thinking about which cards will work together (however successful they are at that) then the chances are they're just probably not going to like dominion. I wouldn't worry about people only buying money because jeez, nobody figures that out, they just see all the cool cards and buy them loads.
Just observing them for a couple games and not playing yourself is definitely a good plan I think, just get a beer/cup of tea and sort of "commentate" on the game as it goes along - it always a little intimidating or frustrating or just a bit weird knowing that the other person you're playing knows exactly what's going on, and getting advice from a competitor ("yeah, you should get a gold, that's the right move") is odd, but not so much from an observer.
The starting set is, frankly (and in some ways unfortunately), the ideal set to start things with I'm afraid. It presents some nice choices with some slightly more advanced cards (remodel and mine - ok they're not advanced as such, but it takes everyone a couple of plays of them to be comfortable), yet at the same time it doesn't give away all that dominion has to offer. The slow reveal new mechanics was one of the best things about dominion, and I think just having access to all of it on Isotropic can spoil that a bit. However, I have to be wary of my own advice there and not try too hard to guide people's experience, just let them explore. I think also heavy Isotropic players should consider that for some reason it's just different in real life - there's less clarity and more fumbling, more chatting, more distractions etc and the game flows slightly differently.
Overall, I reckon the worst thing you can do is power-crush them into the dirt, and probably second is to lecture them too hard on what's good and what's not, forgetting you had your own learning curve.
PS - whoever mentioned masquerade is dead right - that's a great early game card as it keeps everyone involved and engaged. Also, cursing cards are often not popular at all. It's fun to curse, but even for players like me who've played loads it can be pretty draining when the game drags out as you feel a bit powerless due to heavy cursing. Almost everyone I play with breathes a sigh of relief when heavy cursing/thief-tastic/militia-goon game has finished. It's ok now and then, just not all the time, and even less so for new players.