No brain radiation until she can walk. Is this normal?

I apologize before hand if this is the wrong forum. I wasn't quite sure where to put this. My father's girlfriend, who received her initial breast cancer diagnosis shortly before I did, fell ill a few weeks ago and it was subsequently discovered that she has multiple brain mets. She had surgery during which they were able to remove two of them, and then experienced a number of setbacks for which she is currently in rehab. She is cognitively intact and can follow commands, though she is not currently ambulatory. She was supposed to start brain radiation but my father says that he has been told that they won't start treatment until she can walk. Is this a standard requirement? It sounds a little strange to me. She had difficulty walking before surgery, which I suspect was due both to dizziness and balance issues from the brain mets, and I'm concerned she does not have the time become ambulatory before the window of benefit of brain radiation closes.