I understand where you are coming from. You did all the work while your roommate watched TV, yet they expect to reap the benefits of your labor. While I think it's a bit dishonest to hide the price adjustment from your roommate, it's also unreasonable for your roommate expect the full split difference if they played no part in getting the adjustment. I think it depends on your relationship: if you are good friends and there's been give and take in the past, you should just split the difference because you'd expect your friend to do the same for you (you put in all that time and effort for mutual benefit). If your acquaintance is always a penny pincher or they always give you the shaft, I wouldn't refund a penny (and kick myself for allowing them to get to the folio first).

Variations of the same dilemma:
1. You have a 20% off discount code. Do you share the savings?
2. You get a special discounted rate by redeeming a certain number of FFP points. Do you split the discount rate or the full rack rate? How do you compensate for using your points for the lower rate?
3. You redeem mileage for free nights. Do you ask the other person to pay half the regular rate?
4. You receive an upgraded room due to your status at check-in. Do you charge the person more?
5. You used a referral code when you booked and you will get 10% back in 1 month. Do you give the other person any of the kickback?