Hi jolyon! I am a registered nurse in the UK and got my registration last year. If she already got her IELTS all she needs to do is to contact NMC and ask for a registration pack which will be posted to her. Then she needs to complete the application requirements. Once completed she will be given an NMC registration letter which she can then use to apply for her ONP. She doesn't need to be employed in order to enroll for an ONP programme because the school can provide places where you can have your training. But it is advisable to be employed because it will be easier for her in the end. You can contact your nearest NHS hospital or you can contact City University in London because they offer courses even if you are not living in London they will refer you to the nearest NHS hospital where you are located. If she wants to start soon her main priority will be the registration for the NMC she can start doing this even on fiance visa.
Hope this helps.