1 - Is there a specific way she feels lacking in confidence? Is it a very all-encompassing lack of confidence or is it centered on something more specific, like lacking confidence in social settings (as opposed to, say, feeling like she's destined to fail at cooking if she tries a new recipe)?
2 - What religious views does she subscribe to? My advice would be to try to fill her with a sense of power from a deity or force that she identifies with. For example, with my Christian friends it's the good ol' Holy Spirit, which is the "part" of God knowable in one's daily life and which tends to most directly interact with us. For my Thelemic friends I'd suggest invoking Ra-Hoor-Khuit, with special emphasis on scripture like "Fear not at all; fear neither men nor Fates, nor gods, nor anything. Money fear not, nor laughter of the folk folly, nor any other power in heaven or upon the earth or under the earth. Nu is your refuge as Hadit your light; and I am the strength, force, vigour, of your arms."
Even if she's not participating in the spell or doesn't even know you're casting it, she'll still be the one receiving its energies, so it's best to send them in a format that will be most familiar and comfortable to her, and the easiest way to do this is to work as close to her own religion as possible.