Clearly, if you answer an ATC call on behalf of a pilot who doesn't respond, that pilot will hear your communication. But there are some actions that may go unnoticed unless you call them out. It could be a simple thing such as leaning out the mixture on the ground or in the air. Or it could be something more critical such as reentering a frequency that was entered incorrectly. But before you make any changes, make sure that you are not making things worse. I once flew with a pilot who, for whatever reason, changed the frequency I was using without saying anything. Fortunately I noticed fairly quickly, but it could have been a very serious problem had we been flying IFR.