My team,
AIM Robotics, was created because several homeschoolers wanted to participate, and couldn't get on a high school team. We're community based, so we have no rules about membership. We have several homeschoolers on the team, as well as some from schools without teams.
It's been my experience that most high school teams won't allow homeschoolers. If your friend can't find a team to join, she could start a community based one. There aren't enough homeschool and community teams in FIRST.