I used to have an issue with chin splints and occasional foot pain. I'vee been using a product called Superfeet and they're fantastic. They give great arch support which helps with pronation and keeps the foot from straining. I've had no problems since and recommend them to my clients. Most good sporting goods should carry them. Also, have her do towel grabs with bare feet to strengthen the Abductor Digiti. Just throw a towel on the floor and use your toes to grab and lift it.

Personal Trainer Los Angeles

Running with severe overpronation and flat feet is a sure way to get serious injury, im speaking from personal experience (3 stress fractures). She needs to see an orthopedist and get custom made orthodics, store bought ones might do the trick, but while training for a marathon it would be safer to see a professional, and custom ones though more expensive last a lot longer, about 5-10 years and work better. Towl grabs are a good way to stregnthen the lower leg muscles, she should also focus on stretching the lower leg. As runners we automatically stretch the hamstrings, but foot and lower leg pain means she should really start adding in a lot of stretches for the gastroc. step strechers are great for this, but the cheap way is to balance on the edge of a stair and lower the heel, at least 5x 30 second sets for each leg.