Question regarding go up a level and endgame

[boomshay]

4 player game

Players:
Rich combat level 7
Michael level 8 elf, combat level 10
Matt
Becky

Rich kicks down the door to find a level 9 monster
Michael offers to help for 1 treasure (no one realizes he's an elf)
Rich agrees
Option is given to Matt and Becky to interfere, both shake their head.

Michael plays go up a level card and points out that he is an elf for the victory.

Matt then tries to play a BO card (would force Rich to play combat with no helpers)
Party agrees Matt was too late because they already had the option to interfere and passed.

Party gives Michael the victory due to everyone's ignorance of his elf card, but would like a clear answer on if the GUAL card was played fairly.