Modified many frames and would say that it isn't worth it - airbox/seat/tank alignment problems - just too much work. Steel is better, aluminum frames are either 6000 series (will require heat treating after welding) or 7000 series (self aging but hard/brittle difficult to work with). Why not get her a KX 100? If you are determined to lower a full sized bike use spacers in the suspension, cut down seat and a lowering link (all reversible)?