Vaginal bleeding? I am assuming you mean like menstrual blood and not like a cut in the vagina, just to be clear?
Bleeding from the uterus is a completely normal and very common side effect of Depo-Provera, especially in the first few months that one begins to use Depo Provera. In fact, it's the most common reason women stop using Depo-Provera during the first year of use, due to irregular bleeding (spotting) and/or prolonged bleeding. It's especially common during the first 3 months after getting your first Depo-Provera shot. Heavy abnormal uterine bleeding can occur or disruptions in the menstrual cycle can last long after an injection. After getting several shots, many women find that their periods decrease or even go away completely though.
I had menstrual-like bleeding constantly for over 3 months once when I was on Depo-Provera, and I wasn't using meth or anything similar.
She should definitely stop using the meth though and go to a doctor if the bleeding lasts for a long time or is extremely heavy.
I am not aware of any interaction between methamphetamine and Depo-Provera. Inducers of the liver enzyme CYP3A4 may decrease the blood levels and therefore the effectiveness of medroxyprogesterone acetate, the drug that is in Depo-Provera, because it is primarily metabolized by that enzyme. MDMA can inhibit this enzyme, but as far as I know, methamphetamine does not. Many other drugs do though.
But as crashing said, using meth plus the fact that she is bleeding can lower vitamin and mineral levels. When I had constant bleeding after getting the Depo shot the doctor said there was nothing they could do except maybe try giving me some birth control pills to take on top of the Depo-Provera, since the Depo shot is not reversible, it's in your system for months. But maybe other doctors might have some better ideas.
Your friend should take some vitamins and minerals, starting with an iron supplement and a B-Complex.
ETA: I would also take a supplement of calcium, magnesium and Vit D3, because the Depo shot causes bone loss.