Because there is no formally recognized dental specialty known as "implant specialist", I will assume for the purpose of my answer that the dentist providing implant service is a prosthodontist, which is the true specialty which is most closely aligned with implant service. Please correct me if this impression is in error. (For example, neither an oral surgeon, a periodontist, or simply a general practitioner who does extensive implant work can properly characterize himself as an "implant specialist".)
I will also assume that one of the four implants which formerly supported and retained your wife's complete upper denture has either loosened, fallen out, become infected, or failed in some way. What is unclear from your narrative, but not particularly germane to my answer, is whether the proposed bone grafting is intended to shore up the failing implant fixture, create a suitable site for the placement of additional implant fixture(s), or improve the contour of the edentulous (toothless) gum ridges to better support the denture base.
The primary issue, then, is whether there is a suitable alternative to an autologous bone graft-- i.e., a graft from bone tissue harvested from elsewhere in your wife's body. The answer is uncertain, and depends on several issues directly and indirectly related to the methodology itself.
There are several different types of bone grafts: an autologous graft (transplantation of bone from within the same patient), a homologous graft, or "allograft" (graft derived from another person), a xenograft (from tissue derived from an animal), or a synthetic graft. I have listed these four different types of grafts in order of decreasing similarity to the tissues in which the graft will be placed, and this is significant: the human body is more likely to accept tissue that is most like its own, and more likely to recognize tissue as foreign as its composition increasingly differs from its local environment. You do not specify the type of graft which had previously failed, and this is relevant to the selection of another approach. For example, if the previous graft was a synthetic or a xenograft, the superior compatibility of an autologous graft might make sense. On the other hand, the failure of an allograft, if that's what it was, might not leave all that much room for improvement in the prognostic outlook.
Of course, in addition to the selection of an appropriate graft material, proper technique also figures into whether a graft succeeds or fails. I cannot know the qualifications of your wife's "implant specialist", but this variable needs to be carefully considered-- perhaps by means of an independent second opinion.
Autologous dental bone grafts, when they are not derived from elsewhere in the mouth, are typically sourced from either a rib or the iliac crest (hip). Your wife's prior radiation therapy would rule out the use of a rib, which leaves only the iliac crest, if that treatment otherwise makes sense.
It is true, as you say, that "... medicine is able to restore a face or transplant kidneys, bone marrow etc.
". However, it would be more accurate to say that medicine is sometimes
able to restore a face or transplant kidneys, and bone marrow. There is no therapeutic method that offers a 100% success rate, and bone grafting is no exception to this rule. When bone grafts fail, they sometimes fail due to intrinsic host factors rather than errors in technique or methodology. This is not to say that there isn't some approach out there that might help your wife, but some consideration must be given to the possibility that your wife may not be a suitable candidate for bone grafting. Bone physiology is deceptively complex, with many biomolecular pathways that are only beginning to be clarified. There are some patients who, for reasons that are not always clear, cannot successfully accept bone grafts. The reasons may be due to obvious systemic disease, such as diabetes, metabolic bone disease, or changes secondary to antiresorptive medication, or more subtle issues that are never identified.
For this reason, your wife and her doctor(s) must decide just how much uncertainty they are willing to assume with repeat attempts at bone grafting. There is no doubt that an autologous graft offers the greatest probability of success, although it unavoidably entails additional morbidity at the donor site. There are no certain answers here.
However, I must come back to the credentials of the dentist providing the implant service, because the best type of specialist would be a prosthodontist. If there is any doubt as to the qualifications of your wife's current dentist, she owes it to herself to seek out a second opinion from a board certified prosthodontist. Not only will a specialist in this discipline be best qualified to evaluate the services she's had to date, but a prosthodontist will be able to determine whether other treatment approaches might be available that would avoid additional grafting altogether, and render the entire matter of bone grafting irrelevant.
To find contact information for a prosthodontist near you, consult the online directory of the American College of Prosthodontists
Hope this helps...