First, show the Language Bar, which you do by right-click on the taskbar, point to Toolbars, and then click Language bar. Since you've already added French under Regional and Language Options, the Language Bar should allow the selection of English (EN) and French (FR). Second, go into Word and select the Tools menu, then Language, then Set Language. Check the box that says "Detect language automatically".
Now when she's in Word and she wants to type in English, she'll click the Language Bar and set EN. When she wants to type in French, she'll click the Language Bar and set FR. Word will automatically use the correct dictionary (English or French) to do the spell check. Regards, Joe