Hi zaphod,
It is nice to know that you are trying to look after your Mum and want the best for her.
Please do not alter the dose but get some guidance from the prescriber. You need to be careful with changing the dosage as it can cause problems and over time it may even make the depression worse.
We are not medically qualified to answer your question and I doubt that one forgotten dose is going to lift her depression. Monday is not far away to consult her G.P.