You failed to point out that Monty was in fact playing at the rock concert, and therefore it was taking place in his very flat.
This was one of the reasons Nautilus was so drunk because he hated rock music and Monty loved it. Although they didn't really like each other much they had agreed on an internet website to live together for at least 6 months.
only 4 months have passed and every other weekend Monty would hold a rock concert in the flat. But the most important thing is - Monty never knew that nautilus had a wig.