The thing about this rule is that it is not automatic; this is a common misconception about it and the repetition draw. If either player wants the draw, they must claim it by pressing the appropriate button.
Unfortunately, you may need to let them play it out. Informing them about this option and how to do it could be misconstrued as "external assistance," especially since a mate is theoretically forceable.
Is the rule that after 50 moves without a capture or pawn move, the game automatically ends in a draw upheld in chess.com online games? I'm currently hosting a tournament in which the last caputure (and subsequently pawn move) took place on move 49, one player has night and bishop and is trying to mate the opponents lone king, but isn't having any luck. Now they are at move 119, 10 moves past the point the draw should have been reached. What can be done about this? Let them play it out until mate or they agree to draw it?