i really doubt she'll run it. she can certainly run/walk it...

I've done a few marathons on 15-mile-per-week training, but I had an extensive base and tens of marathons behind me. If her experience is limited to "light running" I doubt she can train for a running marathon in 13 weeks. If I were her trainer I would discourage her from even doing a run/walk. Many marathon courses close at 6 hours and it sounds like she'll be well over that. A painful failure does not foster health nor enjoyment of the sport. I suspect it would be a very discouraging experience - even if she makes it. At formative stages, runners should be learning to like running for the sake of running. The point that should be emphasized when working with newbies is creating a healthy lifestyle, not an ill-advised, short-term goal. I suggest you recommend a more realistic goal - like a 10k or a half M.