OP hasn't said whether or not her friend is going to actually do the marathon, just that she has secured a place. She's also said that she's going to support her.
There's been some brilliant advice on training for this on the thread peppered with the usual non-advice just putting down.
I think it's a fair assumption that she is presently planning to do a marathon, given she applied for a place, got the place, and told her friends she'd got the place.
I'm going with the more upbeat positivity rather than the judgemental clap-trap that some posters are so fond of. I'm also assuming that OP's friend has a functioning brain and will do what is best for herself.
Functioning brains can only make decisions off the information they have or are given. If she's fed 'upbeat positivity', she may only realise that her plan is overly-ambitious after she's tried to work through the first injury. Which might be irreversible damage.
Now, perhaps right now, she's googling running advice and learning about all the ways you can screw a joint for life through hasty over-training, but perhaps she hasn't yet, and won't do until the OP tactfully tells her to do a bit of research.
We all like to think we've got functioning brains, but does having a brain alone allow you to diagnose or prevent medical complaints? Or do you need a bit of education to go with the brain?
I do a reasonable amount of sport, and it is my observation that novice adults don't fully comprehend how easy it is to damage yourself as an adult through sheer over-enthusiasm until they've done it at least once. If they're lucky, it's something that heals.