If I were you, I would pay for their stay at the rate including breakfast (since there is no way they will be able to eat outside for 5 euros for two breakfasts), apply the bonus buck, and get your brother to open a Marriott account if they have not already. If your brother does not have a Marriott account, you can refer him for an account using Marriott's referral program here, which will net you and him each an extra 1,000 points for the stay.

You should be able to make a reservation in the name of the account holder staying in Prague (your brother) by calling the platinum reservation line and pay for it with your own credit card. By using the referral program, you will get some Marriott points (assuming he does not have a Marriott account), and you can get your brother to transfer his points to you later if your brother does not travel with Marriott much. In any case, Marriott does not close accounts so you should not worry if your brother does not stay at Marriott again for a while.