Having shared many a Wardroom with Junior Surg Lts, I'd offer the following:
It's quite easy to move Reserve - Regular. The Medical training (and 90% of the 'Mil' training) is exactly the same, so there's no problem there.
She may wish to hold off joining the RAF until she's entered Specialist Training (or doesn't want to do it in the RAF, i.e. just do 'General Duties' Doctoring for a couple of years). The Defence Medical Service's requirement for a particular speciality will over-rule her own desires: their training spaces are very limited in many 'popular' areas.
Once she starts her specialist training, she will, by most measures, be a civvy doctor until she qualifies as a Consultant. At that point, she'll spend about 75% as a Mil Dr, and the rest of the time keeping up her skills as a civvy.
Good luck for it all!