However, as I understand it, and I'm not directly involved with med school admissions, the main issue is with the fitness to practice rules, and, I suspect, particularly in terms of the ability to engage empathically with patients and work effectively with colleagues. They shouldn't dismiss someone out of hand because of having Aspergers, but will want to make their own evaluation
Yes, this would be my first concern. Reasonable adjustments have to be "reasonable" both ways - academic progression can't be undermined, nor can fitness to practice.
But universities are likely to have good systems for assessing this.
The advantage of declaring is that support and reasonable adjustment will be there from the start. No mitigation can be given without independent medical certification of a diagnosis. And, at my institution anyway, mitigation can't be sought retrospectively.
Lots to think about. It would be worth talking to student services/learning disabilities counsellors at universities your DD may want to apply to.