Three years ago, an American visitor to this site started posting loads of questions about his forthcoming visit to England. I ended up corresponding with him and then meeting him at Waterloo station, to travel on Eurostar together and to give him a guided tour of Paris. He had no problems (without a visa) with passport control at either end.
Because rules can change, I've checked the official French government website for up-to-date information. It is quite clearly stated that US passport holders do
not require a visa to enter France for stays of less than 90 days:
http://www.diplomatie.gouv.fr/en/france_159/en tering-france_2045/getting-visa_2046/do-you-ne ed-visa_1559.html
Your friend should have no problems whatsoever. The only difference between her travel and yours is that she might be required to complete a 'landing card' when returning to the UK.
Chris