How to convince labmate to graduate? Graduating, in a 2 person research group?
How many graduate students your adviser attracts at any given time to his or her lab isn't your responsibility. Your responsibility is to yourself and your career. If you think it is time to graduate and if you think you have sufficient material to defend your thesis, then it is time to graduate.

Leave the details of how to run the lab after your departure to your adviser. It might make sense to discuss your hopes and plans to graduate at a particular time with your adviser well in advance (it's always a good idea to communicate!) so that she can plan accordingly. But, this planning is her job, not yours, and it shouldn't be a factor in your considerations.