I don't think the exchange rate will move more than 10 % either way. Assuming worst case scenario, a euro will probably equal $1.60 by July 2008. Including the $50 bonus buck, you could buy your brother a $400 gift card, which should cover the room and tax and leave some room for loss from the hotel's exchange rate. Your brother would only be short if the euro is greater than $1.58 on the interbank rate.

Is it worth it for you to pick up a little more than 5,000 Marriott points by doing this (including the sign up bonuses)? That is a question you have to ask yourself.