Title IX might apply to her. It was enacted in 1972 to promote gender equity in sports.
I am not sure if because they offer a flag program, they don't have to allow girls to play tackle or what, but this link might help.
http://www.usatoday.com/sports/college/stories/2002-06-19-title-ix-cover.htmI played tackle football in Germany and here. There are now women's tackle leagues in many cities across the US.
Growing up near DC, football is ingrained in my personality. I would say the best way to learn the rules is by watching it. You can read the rules, but without seeing them in action, it will be hard to apply them to the game. Watch ESPN when they break down games, talk about different plays, and penalties. If you have the NFL network, watch that. College rules only differ slightly from the NFL, I would suggest watching both.
There are some great books on football. Remember, though, knowing the rules in itself doesn't make a player or coach. If you contact the league she is going to play in, I would ask for a rule book, or where you can get one. The main rules will come through watching the game.
Here are some links, and you might get the book "How to play football the NFL way".
http://www.knowledgehound.com/topics/football.htmhttp://www.womensprofootball.com/http://www.womensfootballcentral.com/http://www.iwflsports.com/And here is a Washington Post video series on their local Women's team, the DC Divas.
http://www.washingtonpost.com/wp-dyn/content/custom/2005/07/06/CU2005070601691.html?nav=lb