Hmmm... Let me play the devil's advocate here...
If she is as good as the other cellos, what would be your goal for her practice? To be more advanced? To make sure you get your money's worth? :-)
I would think that as long as she is not falling behind, and is having fun, letting her be is a real option. She can enjoy making music with the cello for a year without investing much effort in it, and decide later on whether she wants to be more serious in middle school or go back to just playing piano.
My DD (5th grade) does a third instrument at school and she also doesn't practice. She says she is one of the best in her section and she is playing this third instrument just for this year. She works hard on piano and violin. I think it's fair.
My DS back in 5th grade played percussion in school (also his third instrument). He also didn't practice but was really ahead of the other percussionists. My policy at that time was "as long as the teacher doesn't complain, everything would be just fine."
