Everything here will depend on the glue that was used. Something you might try. If the top is only 22", the shelf is probably less. Wet it throughly,(soak) and then put it the oven at a low temp (100 to 175 degrees) and check it every now then using a jig and clamps to try to pull each piece back on plane. The tongue and groove joint will be hard to separate. If plain white glue was used, it will soften up some, but the yellow wood glues (while not considered water proof) are not so accomodating. This might work.
If you've got access to a wood shop you may want to just saw it apart and install a piece to fill the space left by the saw kerf, then refinish. No danger of water damage or warpage this way. I think that would be my choice.

It stimulates creativity to bounce around ideas and to see them in writing. The suggestion to saw apart may well be the only real solution, but I am reserving it as the last resort. The lower shelf is not exactly rectangular, it is sawed with curves in the outside shape. The top is rectangular. The curves will complicate repairs, especially adding a narrow strip. One way of implementing Wayne's suggestion might be to saw the shelf after carefully measuring and recording the measurements. Then glue a wider piece to one of the shelf halves, then rip that (now too wide) half to the final desired width by sawing off the proper amount of the added piece. That would necessitate two gluing operations to restore the shelf. I might even use biscuits both times to help with alignment. Any other ideas?

Before you saw it, make a cardboard pattern of it. (You could shift the piece into alignment as you were drawing to compensate for the misalignment and make the pattern true)Then saw out a defined amount, add the new piece, glue it back in and use the pattern to reestablish the original form.
Have you thought about making a whole new shelf from scratch?