DHA is found in fish, it is not found in vegetable oils like flax. dont get me wrong, Flax oil is great to take and will be helpful, but you need the DHA on top of that.
a great thing for her to eat would be sardines. they are high in DHA, vitamin D, and calcium, but low in heavy metals. in fact they have neglible mercury content. one of the only fish from the ocean that is true of.
Phosphatidylserine is another supplement to look into, as well as vinpocetine.