Hmmm. Security. How much money you got to spend?
As for catching
someone in the act:
1) Why?
2) Dangerous, and could be viewed as an invision of privacy & therefore human rights.
3) Entrapment.
4) Why?
The guy was caught red handed on CCTV you said. So why?
Right, that out of the way. Items needed:
Duct Tape
Maria's bag (empty of everything valuable)
Rape Alarm
You & maria agree a specific point to place the bag. When no-one's looking, Maria places the bag in the quietest spot you can find in the library
Put the rape alarm under the bag, and tie the rape alarm string to the underside of the bag. Leave the bag unfastened, but closed, with something tempting (empty wallet?) sort of sticking out a bit (but enough that you have to open the bag to get it out).
Use the Duct Tape to secure the rape alarm in place.
Maria walks away "looking for a book" (asking library staff rather loudly as she does).
Victim spies bag (hopefully) then
CACKS themselves when the rape alarm goes off in a quiet library. Just before it does, you take a picture.
Walla. *takes a bow*
P.S. If you want something more elaborate, something can be arranged, I design & build security systems for a friend that owns his own security firm.