It may depend on how long he's been there, how obvious he is about what he's doing, and the space available. The following circumstances might get him kicked out:
-causing a ruckus (naturally)
-disturbing/bothering the actual participants
-bothering the researchers who are trying to do their jobs
-sessions are done for the day and they're kicking everyone from the space to close up shop

If the scientists happen to notice that he's just hanging around all day and not contributing, they would likely ask him what he's doing there, and invite him to try a session for himself. They may also request he leaves the room for a while if there are enough volunteers waiting that they want the space cleared for people who are taking part in the research.

Otherwise, so long as he's not causing problems, he won't get much trouble for watching.