On what grounds can a lease be terminated?

Your question is a little unclear as to who the landlord is.  If your son and his GF are both tenant on a lease, then your son is not able to move out unless the 2 girls that moved in are going to pay his portion of the rent as he will be liable for the money based on his signature on the lease.  In other words, the 2 girls that moved in are not on the lease and are more or less not liable to the landlord for payment.  if your son's GF is the landlord, then she can agree to let him out of the lease by agreement.  Make sure there is a written letter from her and signed by her stating that he is no longer liable for the rental payments under the lease.  If not, she may try to get money from him if they break up.