First, I would say you are right to consider balance issues before doing radiation. At 1 cm, there should be time to get it sorted out.
If possible, I would ask one of the doctors to refer her to a vestibular lab, and get a set of tests run to assess the balance nerve function, including ENG. This can help determine the extent and nature of balance nerve dysfunction, including which branches are affected. The results may help a doctor advise you on treatment.
Radiation treatment does not directly affect balance function one way or the other. That is in contrast to surgery, which typically includes snipping the balance nerve on the AN side, leaving you with one good balance nerve on the other side.
In many cases, the balance nerve signal on the AN side either remains usable, or fades away over time on its own. The brain can readjust to this quite well, with maybe a few bumps in the road along the way. When that is the case, radiation will work out fine.
However, if the balance nerve on the AN side ends up actively sending out jumbled signals after radiation, you could get stuck with serious vertigo problems. That's a scenario where surgery would be preferable, since it would snip the nerve and shut it down for good.
In my own case, I have a mild loss of balance nerve function on the lower branch, which makes me wobble a bit when making sharp turns, but doesn't affect me otherwise. That has remained the same before and after radiation treatment, and it is not expected to become a problem.
Best wishes,
Steve