If Ken is training that hard, he should have rest days with easy rides. Barbie should join him on a rest day easy ride, preferably the day after the hardest ride he does all week. Ken should also realize that allowing Barbie to set the pace ensures that he will not be over training.
As heltonbiker has mentioned in his comment, this can also be done on the same day, e.g. Ken can train hard in the morning and ride with Barbie in the afternoon.
If Ken really feels that he would like to go longer, he should do one 20km loop with Barbie and then a second by himself while Barbie gets on with the rest of her day.