You did absolutely the right thing telling her that reiki might affect her meds. BTW if she had gone ahead with the treatment, you then could have intended that the reiki be given with the medication taken into consideration. I have done that in the past, and that mitigated side effects.
BTW I had a sort of similar situation where I was asked to give reiki to a 93-year-old woman who was very depressed and who had, according to her concerned friends, "given up on life". I said I'd be happy to give reiki, but I had to warn the concerned friends that reiki did what was best in any given situation and that it was possible that in this situation it might have the result of easing their friend's passing, if that's what was meant to happen. After that the reiki treatment never happened, and I can certainly understand why the concerned friends backed off, but I must also say that I was relieved, because I felt uneasy with the idea of maybe unwittingly contributing to someone's death, even if it was meant to happen.
It's quite a quandary, because then of course I felt bad about not helping someone who clearly needed help ...
Just some thoughts ...
Judith