As someone who has given "free window clean" vouchers away to raffles, fete's & charity's before, I don't think you can blame the old lass for utilising her prize, A free clean is a clean off her outgoings,
What you can be disappointed in is her failure to pre warn you that she may want to miss, this was ignorant of her.
It sounds like the firm who gave it away were using it as a way to upsell other services they offer as she already has a good window cleaner you would seem unlikely to lose the jobs of her or her daughter.