First thing first: Barbie should NEVER suffer a penalty because of Ken's preferences or athletic level. It is up to Ken to solve by himself the problem of making Barbie enjoy the ride. Leaving her alone is not fine, unless she explicitly, truly says it is.
One possibility could be to add weight to Ken's bike, or maybe Ken could choose, among his many bikes, the one less efficient, be it because of weight, aerodynamic position, or rolling resistence. Of course, if he has a blazing racing bike, he could very well fit it so that Barbie can ride with less effort (and thus faster).
If Barbie and Ken are in a firm relationship and their mood is well tuned, riding a tandem is a garanteed success. Take a well-paved, fair traffic road and Barbie will love to go above 20mph while Ken beat the sh*t out of his legs.
Let's go further in the future: if they happen to have a CHILD some day, of course Ken is going to carry her around on his bike. Around four years old, the "average speed" of Barbie and Ken+Child should be quite similar, and beyond that age Ken is starting to fall behind... Above eight years old, or even sooner, the kid can eventually become the tandem stoker, while Ken is the captain and Barbie goes happily along with her own solo bike.
It is important to mention that every regularly performed physical activity develops endurance, then perhaps if Ken is patient, and surely if he gives up a bit the "every ride is a workout" paradigm, Barbie can improve, then Ken doesn't have to wait so often, neither she has to keep riding only 20km. But ONLY if he is patient.
(That pretty much describes my recently completed five years of marriage, with a stepson now 11 years old. I believe even a steady date can benefit somehow with this tips. Of course lots of times things go wrong, though...)