JSharples
Welcome to the Forum. There seems to be two aspects to your question about Boer War recruitment.
(a) If a male civilian who lived in the UK, was of military age and wished to do active service in South Africa circa 1901, he could apply to enlist in the Imperial Yeomanry. Previous service in the Militia or Volunteers would have been a definite advantage to such an enlistment. At the conclusion of his enlistment period, he could apply for a local discharge in South Africa and look for opportunities there.
(b) If, however, a male civilian who lived in the UK, was of the general age bracket, and wished to join one of the Irregular Corps who were recruiting in South Africa, my understanding is that he would have to find his own way there. And of course, pass the necessary tests before acceptance.
That prospective journey may not have been as difficult as it sounds. At that time, from all over the Empire, potential recruits who missed out on a ballot or were just looking for adventure, could sign on in one or other of the Transport ships as Horse Handlers or travel as "indulgence passengers". Or just pay their own way.
Other Forum members may be able to assist further here; however, I have never heard of - say, Thorneycroft's Mounted Infantry - having recruiting drives in the UK.
Earlier in the Boer War, there was some recruiting for civilians with special qualifications for the Lovat Scouts (Special Highland Scouting Corps) and possibly Loch's Horse. Exceptions to the rule, I would say.
If you could give us your GF's full name and any details of service, etc., Forum members may be able to assist further.
Good luck
IL.