It sounds almost like your code may be fine, but gravity is pulling the arm down once power to the motor is removed.
Try using a PID algorithm (check the whitepapers if you need help on this) to keep the position static (this would require a sensor such as a potentiometer, however; limit switches wouldn't be enough). An alternate solution would be instead of turning the motor off when it reaches the top limit, to just apply a small constant force with the motor to keep the arm stable. Of course, this would not help if someone tries to push down on the arm or the arm's weight changes.
If you were using a FRC control system instead of VEX, a third *possible* solution that doesn't even involve programming would be to place the Victor into "brake" mode instead of "coast." Again, this is for FRC; I don't think VEX motors have this option.
Hope this helps. Try having your team member post his/her code too, it helps us when solving problems to know where you're starting from.
--Ryan