Regarding performance: This is the rater’s responsibility.
Regarding the PT Issue:
Ideally the CSM’s rater should have the frank discussion with the CSM and it should be in writing if need be. Failing to take a PT test tends to be an indicator of a larger issue. If the XO is having this discussion as a professional reminder (verbal) or perhaps as a gentle prod from the BNCDR, meaning the BNCDR has asked the XO to have the discussion to avoid embarrassment to the CSM, I would consider it acceptable and within the XO’s scope. Note: these are not formal written counseling sessions.
However, since an XO does out rank the CSM and is within the Chain of Command he could exercise general military authority. He/she could in fact formally counsel the CSM, but this would be outside of tradition and normal scope. Before conducting a counseling session I would encourage the XO to have the approval of the BNCDR at a minimum and as a courtesy inform the BDE CSM.
Having to have this discussion with a CSM is disturbing. It is extremely important to determine the underlying cause. Is it poor performance? Or is it an issue that the CSM is having difficulty facing from a personal or professional standpoint? Once you know the foundation of the problem you can determine the best way to proceed.
Bottom line: the CSM is the standard bearer and is expected to do the right thing even when no one is looking. If he is behind on his PT test and has an attitude about it, initiate an adverse action flag. If there are extenuating circumstances he should get a temporary profile so that he can overcome the specific issue. This issue needs resolution quickly. He needs to take the PT test or go!