Only the cabinet with the backing of the vice president can invoked the 25th amendment, and a letter from the President reverses it.
What they can do is impeach.
I also read the text of the 25thA to see how it is to be used. Section 2 has the President initiating the replacement. Section 3 details joint responsibility between the Vice President and cabinet or “other body as Congress may by law provide.
The VP is a critical actor in the removal of the president. Congress vs not act unilaterally.
One of the section 3 mechanisms has a curious element — Congress must pass a *law*. Laws can be returned (vetoed) by the President!