Chuck, Houston I Am,
You are both technically wrong. Whilst something infinite cannot expand (since it already occupies infinite space, and therefore there is no space for it to expand into), the universe can be infinite. What is expanding is the observable universe, which is finite. Whatever is beyond the observable universe may be infinite, we just don't know.
For your enjoyment, Dr Neil deGrasse Tyson summing up this entire thing in a minute and a half:
http://www.youtube.com/watch?v=yD1qAEeyets
You are both technically wrong. Whilst something infinite cannot expand (since it already occupies infinite space, and therefore there is no space for it to expand into), the universe can be infinite. What is expanding is the observable universe, which is finite. Whatever is beyond the observable universe may be infinite, we just don't know.
For your enjoyment, Dr Neil deGrasse Tyson summing up this entire thing in a minute and a half:
http://www.youtube.com/watch?v=yD1qAEeyets