Best Boston Burger

My sister is running in the Olympic Trials for the Marathon (which is Boston the day before the Boston Marathon.) We have about 15 family and friends traveling into town from California and I need to make a reservation for our group after the race.

My all-American sister has requested a her post-race meal be an "amazing hamburger and french fries." Any suggestions on where to go? We want to celebrate, so I think we'd almost prefer a low-key bar setting to a restaurant. We are staying near the Prudential Center and, since there is a group of us, we don't really want to travel too far.