Where's the logic?

EN and I are pianists at church and we take turns to play. The most cumbersome part is when I have to play for JC. The songs he chose always do not have notes. Many times, JC goes out of tune. EN usually can due damage control to play to the key they are singing to. Sometimes in vain. Especially after singing one song, and if he doesn't want and intro into the next song, then 'koyak" dy. And one time EN played a chord, but they didn't listen. How un-nerving. A week later, he realized that his singing on that Sunday was bad, tell me that it is because EN doesn't know chiinese. What??? Where is the logic? I explained to him nicely that knowing or not knowing chinese has nothing to do with this. To always be safe and get the correct note/key, it is better to have an intro, especially if the 1st song is not in the same key as the 2nd song. He better attend singing and listening classes. I didn't tell EN this. Sigh!