It has been a dismal season for the Quins, who could still end up bottom of the table if results don't go their way.
However Purdham said that the loss of his brother Garry at the hands of a Cumbrian gunman in June still overshadows events on the pitch.
The home tie against Challenge Cup-winners Warrington will see McDermott take charge of the Londoners for the last time - and will also be the final game for several players.
Purdham said Friday is a "big day" with the likes of Danny Orr, Luke Williamson, Louis McCarthy-Scarsbrook and Will Sharp all set to leave the club.
After tonight Purdham's next appearance will be in a Cumbria side to face England at a special game to remember Garry's memory.
And the captain said that the pain of losing his brother "is still there, I don't think it will ever go.
"It just doesn't seem real," he added.