Consider the level of generosity that Aldern displayed before. This was someone who really wanted to be well thought of. Someone like that would never propose splitting two meals seven ways with guests. That's absurd even with a normal host, let alone someone like Aldern. Not when the alternative of inviting us out to an inn or other venue in Magnimar exists.
Even if we did not know this about about Aldern and Iesha, that alone would have given this away.
Book 2, Chapter #2: To the Manor Borne Msg 558:
"They abducted my Iesha. Told me the only way I'd ever see her again is if I did their bidding. They made me eat some of that," he pointed to the fungus on the wall, "and my transformation began. Soon after they took me upstairs to see her." His voice was ragged. "Or what she had become."
"I am sorry for what I did." This to Liseth. "You reminded me so much of her, and I had already learned of her abduction. He is fixated on you. I don't know why. I believe this is yours." He pointed to a white, horse-shaped figurine on his desk. "I don't know why he does anything he does."