Isn't depo provera the injection you have every 3 months? If she had a full dose, which sounds like she had a larger dose, then she has more hormones in her system already when she started the bc pills six weeks later.
I've not had any experience with depo except for my daughter used the injections for birth control but quit because she was having an uncomfortable amount of cramping and bleeding.
If this were my daughter, I'd recommend getting a second opinion or at the very least ask for a pill with a different make up.
It's my understanding that the reason women don't have a period or at least don't usually have a period is that the hormone level stops the uterine wall from building up a lining. Once an egg is implanted the lining becomes thicker and enriched. Common sense seems like you'd want to get rid of the old lining but perhaps there really isn't any old lining and won't be enough lining to produce a period until she's been off the depo for longer. Just a thought.