I certainly understand your concern and frustration. But your mother is going to make this decision and do what is comfortable for her regardless how uncomforable it makes anyone else feel. Just try to trust her judgement and that she has considered all the possibilities with this choice. Have you talked with her about how/why she came to this decision and what she knows about risk/benefits? Maybe if you have this discussion it'll alleviate your fears, or even convince her to make another choice. The bottom line is though that this is ultimately her decision. You have no control over where/how she gives birth. Just try to support her as best you can. I hope all goes well!
Has she had prenatal care and checkups along the way? Hopefully she has and feels reassured that there are no major medical concerns. Many woman want the homebirth experience for a vast array of emotional reasons and/or not wanting to over-medicalize the natural birth process. Ultimately it is her decision, but one option is that her midwife or doula can certainly deliver her baby at a hospital and be her advocate to ensure that her birthing plan is carried out. I will have a midwife present for my own delivery, no docs, but I opted for a hospital delivery for the reassurance that if something does go wrong with myself or the baby, there's medical staff onsite. It's nice that you are so concerned for your mom :0) Try to support whatever decision she comes to.