Question best answered by the cruise line or TA. No one can predict what will be happening in a week or so never mind next summer, With the size of river cruisers, you are in much closer contact than on ocean liners.
Would your friend really want to expose themselves to the possibility of getting covid in a foreign country--something to think about.
We have just booked our 5th AMA cruise, a charter with the American Wine Society for next April. It will be good to get back on the rivers.
Good luck, stay well. Pat