Politely decline.

"Sorry" you say "I wouldn't feel right turning my back on Ulfric" you say. Dunham gives a brief pout. "Ah well, it was just a thought. I'm sure we'd have had lots of fun together, but I commend your loyalty. Ulfric is a lucky man".

You help him break camp quickly, packing his things onto a wagon then quickly make your way back to Ulfric. You arrive just shortly after a visitor: Sir Langley, the knight the squire warned you about.

"Ah, Eric, just in time." says Ulfric "This is Sir Langley. He's come by to pay his respects before our bout tomorrow"

You bow slightly to Sir Langley. Even besides knowing what you know, the man gives you the creeps. His eyes are dark and sunken, and his skin is coarse and tanned. He looks at you with an expression of complete contempt. "Fetch your master his cup boy, and be quick about it" he barks.

You obey, fetching Ulfric's small silver cup from amongst his things. Langley has his own, rather more impressive, gold cup, decorated with filigree. Clearly the years of cheating have paid well. A bottle is thrust into your hands by your guest and a demand that you pour is issued. You hold the glass bottle in your hands, knowing that the crimson liquid within will do more than make Ulfric a little tipsy.