I wonder if she's going to try track as well. The speed needed for soccer often translates just as well to distances like the 800m and 1500m.
Agree with others that lots of our running talent is probably locked up in soccer teams (basketball, too). But also agree that it's a personal decision, and that just having the endurance for running doesn't mean one has the desire mentally to spend long hours running alone. There's of course plenty of exceptions, but many soccer players I know are super fun-loving extroverts, who just aren't going to get the same thrill from a trail run as they do from the soccer team experience.