A lot of good advice here for the less experienced stoker. I found the seat thing and pacing thing made a big difference for me. There is a LOT of good discussions on the women's forum on the seat. Three was the charm for me and I have a Gregg Lemond seat I purchased on e-bay. It seems like the ticket. Also maybe you can get your stoker interested in the clothing aspect of cycling. Most women enjoy shopping, even if it is for bike clothes!!!!!!!!!!!

I was not a much of a cyclist. In fact I made my husband take off the clipless pedals on the mountain bike because of my frequent bruises. I love being clipped in on the tandem because the way we work it I don't get out at all unless its to eat or pee. Suits me just fine. We also enjoy the time together.
He likes it because I have to listen to his directions about pedaling, etc. and I like it because he can't escape me when I talk to him
Enjoy!!