Had a roommate once that wanted to try road biking so I lent them a spare road bike I had and took them for a ride. Before the ride I warned them that it wasn't like any bike they had ridden before and that they could easily get going faster than they wanted to go in situations where they didn't think that possible. I also told her that I might want to go fast now and then and that she should take her own pace and I would wait. The first downhill, I took off, intending to wait at the bottom. She tried to follow at my speed in spite of my warning and got going so fast that she was afraid to hit the brakes. She had stopped pedaling but the bike kept going faster the rest of the way down the hill.
There was much yelling following this, however I told her that I had warned her about the speed and that she should take it easier on the downhills until she was comfortable with faster speeds. Eventually we ended up riding a tandem together for about a year and then she went back to riding a single bike when the tandem was no longer available. She eventually became as comfortable riding downhill if not more comfortable than me. The entire time on the tandem though, I never pushed to go faster than she wanted to go. It takes time to get comfortable with riding fast on the back of a tandem. You might want to try sitting in the stoker seat for a ride just to get an idea of what it's like to be back there.