She could administer the herb during the planetary hour related to that particular herb. You need to check in something like Culpeper's Herbal (loads of copies online) for the planetary rulership. (Leaves and roots may have different rulerships, by the way.)
Planetary hours are the same as ecclesiastic hours. The time between sunrise and sunset is divided by 12 and the first hour is named for the planet of the day (e.g. Venus for Friday). Then each hour is associated with a planet in what's known as Chaldean order - Saturn, Jupiter, Mars, Sun, Venus, Mercury and the Moon. When you reach the end, you start again. When you get to sunset a new division is done between sunset and sunrise. (This is much simpler than it sounds.)
See:
http://www.renaissanceastrology.com/planetaryhoursarticle.html
http://www.astrology.com.tr/planetary-hours.asp will calculate them for you.
The Moon is used for planting and gathering of the herbs, not necessarily administering them. Except she wouldn't administer herbs during a void Moon as they'd be ineffective (
http://www.skyscript.co.uk/gl/void.html) Technically, she should also avoid the Moon in the first fifteen degrees of Libra or Aquarius, end of Libra and start of Scorpio, conjunction, square or opposition Saturn, Jupiter or Mars.
However, the planetary hour is probably good enough for what you want