Your English is great, so no worries there.
19% body fat on a female is a pretty good place to be, on the lower end of a healthy range, if I recall correctly. Lifting weights will help her get a more "toned" look, but she has to go a little heavier every time.
In order to gain muscle you need to lift heavy things (I like Stronglifts 5x5, personally) and you need to eat a lot. Don't do any moderate cardio exercise- no long workouts, no long runs. If you need to run, then do sprints, as hard as you can, once every 7-14 days. Walking is great, however.
I'm not sure if CrossFit is the best place to be if you are trying to gain muscle, but someone else can maybe speak to that.