When he swims over, about a minute after you spoke to Srafen the Soaked. Do exact conversation as described in the walkthrough, though skip the part where you say "I am not an illusion" and right after your "Hail" say "you won't bore me".
That you get like 3 minutes after Dillon swims over, but you can just skip the wait for their conversation & do your part & then just wait... For Froham the Forgotten, who takes his time, so just be patient... /wink