First of all, Markunthar is very definitely a he.
Second, when you get out of the room of the summoning ingredients guy a conversation about some student trying to seduce Markunthar should be triggered.
Then go back to Markunthar's room and there should be a girl waiting outside. Tell her to try and seduce Markunthar by praising his intellect. Then they should leave together.
I hope it's not the girl that's locked inside the laboratory (some of the NPC's seem to wander around at times). If it's really the girl inside, I think you'll have to go back to a previous save (I've had the same problem with the librarian locking himself in once). Maybe it can be solved by going a level down and then up again, but I'm not sure.
