First, for cycling, she's in a base building period. We're using a little bit of intensity in her bike workouts but we're primarily working on cycling form and efficiency. That means that I'm including cadence and form drills in her bike workouts. For example, to improve her cadence, one of her workouts this week includes spin-ups. She should be able to safely pedal at a high cadence outside. Maybe she won't be able to hold the spin-up as long as on the trainer because of a stop sign, but overall, she'll get the same benefit. Another workout includes single-leg pedaling. I've told her to not try this if she rides on the road. In my opinion, for her, that is a little too dangerous to be doing off the trainer.
Second, spending too much time on your trainer can get incredibly boring. If she can alternate between the trainer and the road during a time when she doesn't have a lot of intensity, she'll stay interested in riding her bike and may do more workouts than if I require her to stay on the trainer so that she fits in all of those efficiency drills.
Third, as I mentioned she doesn't have a lot of intensity during this period. So if she rides a little slower to stay safe in the road conditions, that isn't going to negatively impact her training.
We all want to do well but we all do this because we consider our sport, whether cycling, triathlon, or duathlon, to be fun. As you make the decision about whether you ride outdoors or on a trainer in the winter, consider not just what is best for your training plan but what is fun. Unless you have an A priority race early in the season, you should probably choose the fun option. Just make sure that fun option isn't racing your friends in a ride that is far more intense than you need to be riding at this time of year. The last thing you want is to beat all of your training partners in January, and be off the back of the pack at your races in July!