she didn't need a spell. since cho chang's friend told on them, anyone she told would be able to get in the room however, in the film she uses the spell bombarda maxima.
Umbridge DID need a spell, as she obviously didn't know how to enter the Room of Requirement. Marietta told her the DA was in there, but probably not how to gain access. If Umbridge had known how to gain access via the door, I'm sure she would have opted for that as she would have been able to catch more of the DA as they tried to escape. The above is only correct in stating that she used the spell Bombarda Maxima to blow a hole in the wall. - plekkie