First, the cards :
Desolate passage : Each player cannot declare more than 1 attacker or defender in each challenge.
Walder Frey : After at least 1 defender is declared during a challenge, kneel Walder Frey to have him participate on the side that is currently winning the challenge.
So, if I understand the meaning of these two cards, Walder Frey will join the attacker of defender because he haven't been declared as an attacker, i'm I right ?