Hi Darren @Dman74 ,
I know you asked @LievenDV , hope you donāt mind me answering.
Assuming you both use standard tuning - if you play G, C, D, C without a capo, your mate could use a capo in various ways:
Put a capo on fret 2 and play chord shapes F, Bb, C, Bb
or
Put a capo on fret 3 and play chord shapes E, A, B, A
or
Put a capo on fret 5 and play chord shapes D, G, A, G
or
Put a capo on fret 7 and play chord shapes C, F, G, F
in all cases the sound of the chords will in fact be G, C, D, C, even though the shapes are different.
If your mate, as you asked, plays the G, C, D, C chord shapes with a capo on fret 3, the sound of the chords would be A#, D#, E#, D#, and it would not be in the same key and not sound good.
Bonus info: If you play chord shapes A, D with a capo on fret 2, and your mate plays chord shapes G, C with a capo on fret 4, both of you are actually playing B, F#.
Hope it helps.
Jacob