The reality is that perhaps even with all the training in the world, your paces might never match. Putting that expectation before any of you got on bikes in the first place might be a great excuse to re-evaluate the entire ride.
To make her feel better, maybe tell her how amazing it would be if she was your personal support crew on the ride. That way she's involved, helping, there at the start and finish, and she's not hating every minute of the ride.