She said her son was a mechanical engineer so he should very well knowledgeable about dissimilar metals. As long as the valve has a plastic or brass body she is ok. Steel or stainless would be a huge problem since there would be way more galvanic reactivity. Aluminum and brass are way lower reactivity. Even a plastic valve that could take the heat would be fine.
I did similar with an Atwood WH. Replaced the plug with a brass nipple and brass ball valve. Never had any problem. I could unscrew them if I needed to flush the tank.
I screwed the nipple in and screwed the valve onto the nipple. I had to remove the valve handle for clearance so I could turn the valve to tighten everything. Kept the valve handle in the silverware drawer so I didn't loose it and could find it when needed. I even had about a foot of clear tubing with a plastic hose barb i could thread into the ball valve I kept inside the water heater access door to let the WH drain onto the ground instead of running down the inside of the door.