No, I wouldn't let her ride the distance... but not for the reasons you listed. This info is old, but, worth considering....
During 1987 and 1988, an estimated 92,763 emergency room visits were made in the United States for injuries related to horseback riding. Although the greatest number of injuries occurred in the 25-44-year age group, injury rates were highest for 5-24-year-olds, especially for females (Table 1).
Nearly half the injuries occurred at home or on a farm (Table 2). Soft tissue injury (e.g., laceration, contusion, or abrasion) was the most common diagnosis, followed by fracture or dislocation, strain or sprain, and concussion (Table 2). Most injuries to the extremities and trunk involved soft tissue, fractures and dislocations, and strains and sprains. Head and neck injuries were mainly soft tissue (56.9%), concussions (18.5%), and fractures or dislocations (11.0%). The 14,120 fractures to upper extremities represented the single most common site and type of injury.
Compare the risk of horse riding injuries to the risk of what could happen on the road, then decide.
In my opinion, the risk of injuries from horse back rding outweigh the risks involved with her riding a bike 2.5 miles down the road.