With no understudies, not sure there is much you can do.

Tell the actress to save her voice for the show, drink plenty of fluids and get lots of rest!

The saying "the show must go on" certainly comes to mind.

Good luck!

 

A thing that I have seen singers in musical hall [yes - I?m rally that old] & even opera do, which can help! Is to gargle with apple cider vinegar & honey.
I have used it a number of times, it does work. Only  remember to spit it out though & get rid of all the throat rubbish with it. Although it is also quite a good drink, if you use Scrumpy [alcoholic cider] & by adding the honey, your getting fairly close, to producing the same taste as Mead!
The vinegar cleans out & breaks down the mucus, while the honey tends to coat the voice box & soothe the inflamed throat areas.
You can also use pure lemon juice, but the vinegar works better in croaky cases.

Almost half our cast had colds during our musical which closed last weekend.  I found an herbal tea called 'Throat Coat' with Lemon and Echinacea that helped and they liked the taste (I also got a 'Breathe Easy' tea with Licorice in it but they couldn't stand the taste of it).  Any hot beverage with lemon and honey will help

As SM I always have tons of Halls and other assorted cough drops on hand and dole them out backstage.  Make sure your actress warms up her voice gently before the show.  Do you have microphones?  Can you adjust her mic so that she doesn't have to project too much?

But I hear you about small community theatres with no understudies.  I am always terrified someone will be deathly ill or hurt themselves.  Actually, one of our leads did break his knee 2 weeks before opening night and we re-wrote the script to allow for him being on crutches.  He was a trooper as he was in quite a bit of pain.  I had chairs for him to collapse into as soon as he got offstage.  The show must go on, right? 

Thanks K8tt!
I forgot to mention the Cider vinegar/honey works better as a hot gargle & not that bad a beverage either!
When I directed the Man of La Mancha years ago. The female lead ?Aldonza,, pulled the pin on us for a part in another show, a few weeks before the opening. So I replaced her with a young lass from the chorus, who did a great job!
Unfortunately she contracted a throat frog infection, the day of the preview. So I put a radio mic on her & she delivered her dialogue thru the sound system. When she came to singing, another young cast member sang the songs off stage, into a hand held mic & Aldonza gold fished the songs. Worked great & the punters could not pick up any difference in the audio reinforcement of someone else?s voice, even though we announced that this would be the case before went up, to the bums on seats!
Worked fantastically well.
The only other problem was Aldonza advised us on opening night, that she was 4 months pregnant, so we quickly modified the her costume & the rape scene!
Otherwise it was a great show!