No need to apologize. I just hope I can help you get it straightened out. 
when you read a “D” on a piece of sheetmusic, you should play a “D” on your instrument, regardless of what the instrument is. And as I understand it, a C-whistle play’s a C when I finger it like a D on a D-whistle.
Yes, and yes.
To keep it simple:
Consider this for a minute. Remember in elementary music class (or The Sound of Music) when you learned Do Re Mi Fa So La Ti Do? That is the major scale. It always sounds the same, no matter what note you start on, because it always uses the same intervals.
On a whistle, think of the lowest note as Do. This note determines the key you’re in. If you’re using a D whistle, then your Do is D. If you’re using a C whistle, your Do is C. If your friend on the piano is playing in the key of D, then his Do is also going to be D. Make sense?
So to get the idea, don’t think of the notes themselves on the whistle. Just know that the whistle gives you the major scale Do Re Mi Fa So La Ti Do. When you and your friend play in the same key, then your notes will be the same because your Do will be the same note (as will the others in the scale).
when we would both play together, when I use a C-whistle fingering like a D, he would actually play a D and I would a C when reading the same sheetmusic
No. If you are both playing the same tune in the same key, then you’ll be playing the same notes. Don’t let what Peter said about always referring to the notes as if they were on a D whistle confuse you. That happens and it’s understood among whistle players, but the fact of the matter is that you are in fact playing the real notes inherent in the whistle key you’re playing.
Say you play Mary had a Little Lamb with these notes:
XXXXOO
XXXXXO
XXXXXX
If you play that on a D whistle, you’re playing in the key of D, and using the notes D, E and F#. If your friend plays the same tune in the key of D on the piano, he will be using the same notes (D E F#).
If you play that on a C whistle, you’re playing in the key of C, and using the notes C, D and E. If your friend plays the same tune in the key of C (to match your whistle), he will still be using the same notes as you (C D E), because that’s how you play that tune in the key of C.
So, you’re still playing the same notes as your friend.
Sorry if it seems oversimplified, but I’m just trying to get the princple across.
If you and your friend are looking at the same sheet music. Then you should be playing in the same key (and you with a whistle in that key), and you will be playing the same notes.
when we would both play together, when I use a C-whistle fingering like a D, he would actually play a D and I would a C when reading the same sheetmusic
One more thing on this. If you play a C and he plays a D while looking at the same sheet music, then you’re playing the wrong whistle! You need a D. The whole point of matching keys is so you both are playing the same notes - same scale, same key.
You need your Do note to match his Do note.
Does that help at all?
Jason