I live in an apartment with 2 roommates. One of the roommates and I got the apartment originally and told the landlord we want it but the landlord said we needed 3 names for the lease. So we put in a search for a third roommate and from the beginning told her that it was only a year and we had someone interested in the following year. The lease is coming to an end and recently she told us she may want to stay on the lease for a new year but that was never an option for her. The landlord knows that she was put on the lease so we could get the apartment, even if it meant having her as a roommate for the last year.

What is the legal way to let her know she cannot continue living with us? Does the landlord have a say?

We have not approached her yet because we want to make sure we do this right, she is known to cause problems and this worries us.

Yes the landlord can refuse to renew the lease with her. However, he can also choose to allow her to renew. So you need to talk to him.