If she removes the grass she stands a chance of the creek bank eroding her back yard. Any thing that kills can pollute the water. She could use a thick layer of mulch but every time the creek floods it may be washed away.

They could use a string weeder

Even if she kills the grass at the edge, it will soon fill in again from the lawn. Or tall weeds will invade the bare earth.

My suggestion to her is to kill the grass for about 2ft from the creek edge either by smothering lasagna style or with careful application of Round Up, cover the bare area with mulch and install a root barrier between the lawn and mulch. She can then plant things that like a moist, sunny location like Joe Pye Weed, Cardinal flower, Monkey flower, Button Bush, Iris etc etc in the mulched area.