Understand I am not a doctor or in any way medically qualified to offer
a recommendation.  Anything I say must be treated as the sort of advice
one might get, for example, talking to a guy in a bar.

This is not a life-or death decision, as we have said, there is no
evidence that adjuvant radiation reduces mortality, it just reduces the
risk of needing further surgery for a local recurrence.

So the real balance we are looking at is whether she would rather risk
going back under the knife, perhaps at a time when she is less able to
cope with it, or alternatively risk having less ability to cope with
whatever old age may eventually throw at her, and possibly having a
slightly shorter life overall.

Even this last is not as simple as it looks, if one is in a situation
where the long term effects of radiation may shorten your life, there is
a good chance that your quality of life is already pretty degraded, so
the loss of quality life is not as much as it might seem at first sight:
in some cases curtailing suffering might even be an actual benefit.

I would be inclined to agree with your 3rd opinion, that it is too close
to call clinically, and ends up as a personal decision on the basis of
personal factors and how you feel you are going to live in old age.
Look at survival of older relatives.  Look at your economic and
healthcare situation (good health insurance?).  Look at her health
history (smoking, obesity, blood pressure?).  Maybe some of these can
throw some weight on one or other side of the balance.

Tim