Thank you for all the input.
Finally I removed the screw "cleaning" de head hole with an apropiate size drill and then forcing a torx wrench taping with a brass hammer until it "bite" the metal enough to apply the force necesary to turn the screw.
No damage, except the screw itself. I replace all the screws for new ones and bought to my friend the correct size allen wrench for future use.

Thank you again!

Best regards,

Jose M. Fernandez