He has conclusive evidence that it is there, since he attempted to play 27. ... h2. As I remarked on telling him the move was illegal, it was provable that the h-pawn was still on the file. I didn't explain, but the proof runs thus: When Black captured on h6 earlier it was behind his own pawn, so cannot have been the original White h-pawn (not of a White pawn at all, since White's capture on h6 was not preceded by a pawn-capture notification). The only captures on the g-file were on g2, hence also not by the h-pawn. Therefore White's h-pawn has not left the file, and since Black's pawn advanced successfully to h3, there must still be a White pawn on h2.
If I were in WES's shoes I wouldn't take on h2. There has to be a good chance that the King will recapture on g2 in which case it would be able to recapture on h2 after White was told of the capture. I'll update the graphic later, but White has taken with the Bishop.