She may be a late bloomer, however, growing autos and photoperiod plants together will cause two problems -
1) - your photos won't flower at 18/6
and
2) - your autos will not yield very much at 12/12.
Triggering a plant, which could take two weeks or more, by going to 12/12 is possible, but then going back to 18/6 will just cause it to go back into vegetative growth and to cease flowering. Waiting a bit longer to see if she flowers on her own is the best option for now, but if she doesn't show flowering initiation by week 5 or 6 at the latest, you will have decisions to make. Either set up a new grow/flowering space for the (possible) photoperiod plant, taking her out of the tent at exactly the same time each and every day and placing it in complete darkness for 12 hours each and every day until the autos have finished and before the tent space becomes available again, or reducing the lighting in your tent to 12 hours and sacrificing some auto flower production for the sake of photoperiod plant flower production.
So, the ball is in your court with your decision pending. Apart from the options I have suggested, there really isn't anything else that can be done. Someone may have other solutions and I would be most intrigued as to what other suggestions they could offer. You are never too old to learn!
Hope this helps,........
Organoman.