How bad is the "cupping?" Just a bit of give as you walk over it but otherwise level, or literally bowing with no pressure and then really bending if you walk on it?

If it's the former, I would wait until a few weeks without rain and the seal it, see how it goes. I probally would not remove the board to do both sides as that's a lot of work for not much gain. Stain is weather proofing, not really structural support. For that you need wood epoxy, which will run a lot more than lumber.

If it's the latter amount of bowing, probally a full replacement with either deck quailty lumber if she cares, or cheaper, uglier and more weather resistant pressure treated lumber if she doesnt.

You can also try removing a board and flipping it over. It's likely the other side is not nearly as warped if the bowing isn't bad. Then flip them all over and seal the "new" top side.

I am of no help here, but your post made me think of this. https://youtu.be/tbazGVrbN-g