Extra vitamins etc won't help unliess her diet is deficient, but a supplement probably wouldn't do any harm. "Tonics".sometimes used to contain iron, which would only help if she was anaemic. They also used to contain alcohol, which might make her feel better temporarily, but is not medically recommended.
Basically, any sort of supplement will only help if it's treating a known or likely deficiency, and in most cases a change of diet is a better remedy. It can take ages to get over a virus, especially as we get older.