I agree this may end badly. No one likes to be deceived. She can't be reinstated if she is dating a "worldly" man. And if you continue the facade into baptism, you may find yourself very unhappy. What about children one day? Are you going to deceive them too?
I recently found out that just saying-I don't believe that, I'm sorry but I cannot lie to you works better with my JW family than pretending to go along. Sometimes to help with their disappointment, I ask them-would you rather that I lied to you? They always say no.