The FIDE Laws of Chess state the 50-move rule as follows:
9.3 The game is drawn, upon a correct claim by a player having the move, if:
9.3.1 he/she indicates his/her move, which cannot be changed, by writing it on the paper scoresheet or entering it on the electronic scoresheet and declares to the arbiter his/her intention to make this move which will result in the last 50 moves by each player having been made without the movement of any pawn and without any capture, or
9.3.2 the last 50 moves by each player have been completed without the movement of any pawn and without any capture.
Suppose player A makes a legal move such that:
- The last 50 consecutive moves have been made by each player without the movement of any pawn and without any capture
- Player A's move puts player B in checkmate
Can player B invoke 9.3.2 to avoid the loss by checkmate?
EDIT TO ANSWER A QUESTION IN THE COMMENTS
Question: Is it Player A's 50th or 51st move that puts B in checkmate?
It is the 100th half-move (made by player A) that puts player B in checkmate. So in my example, player A makes the even-numbered half-moves (e.g., 100) and player B makes the odd-numbered half-moves. In turn, this means that player B had the first (odd) half-move. So, the move sequence after the last pawn move or capture went:
1. B A 2. B A ... 50. B A#.
Therefore, it is player A's 50th move that checkmates player B.