I think the Police can draw on information about all the circumstances which can interfere with breath tests. I know that inhaled medicines like Salbutamol are an issue, which concerns me: if I were ever breathalysed I would certainly inform the Police at once, before being tested, if I had recently used the inhaler and how long before. But apparently the influence diminishes to an insignificant level after only a few minutes, so I would probably be OK.
So, assuming that in this case, all possible mitigating circumstances have been considered and set aside (and if this woman believes that anything has been overlooked, she should get legal advice at once) - then we are left with the amount of alcohol intake in the previous 24 hours, and the metabolism rate. Both of these are uncertainties.
I'm afraid I conclude that there is only one golden rule. If your job, or your personal circumstances, imply that you may have to drive any time, never put yourself over the limit by alcohol intake. A 'safe' amount for most people would be two units but do not rely on it! Note that I am talking about the limiting amount you can drink at any time, not merely at a time just before you drive. This woman is a nurse who has to drive as part of her job. Social drinking to a risky level is not on, for her.
Two glasses of wine? A most unreliable measure: I have no idea how much a 'glass' is (at least with beer you know it's a pint or half-pint - or some metric equivalent). Many wines nowadays are 14%ABV, so about 140 ml (1/4 pint or one-fifth of a standard bottle) would give you your two units.