That would depend upon what BF% she is NOW. Anything greater than 15% BF and you would NOt see much difference/effects in taking a fat burner.
Also, the FIRST thing you should be looking at is her DIET. If this isn't right, then looking at a supplement won't fix a poor diet.
And make sure she is training appropriately (i.e. lifting weights, not doing endless hours of cardio).
IMO, you DON'T need a fat burner to lose fat. If your diet and exercise is targeted at fat loss, then you will see results right there.