Answer:
First, she notes down her current position as point A.
Then she checks out on the other bank, a big stone (point B) or something (which is not too far from her), she then goes along the bank to the point C so that BC is perpendicular with the bank.
If she can mark down the distance she walked AC and the angle BAC, then she can calculate the width of river (AB) by formula:
BC = AC x tan(BAC)
Hope this helps!
:)
