Bismillahir Rahmanir Rahim.
Allah says in the Quran, “O you who believe enter into Islam fully”.
Marrying someone while they are still considering Islam is not considered a valid union. The Shari’ah advises that we hold firm to fact and abstain from ambiguities. There should be no force or compulsion in choosing ones faith. Therefore it is better to wait until she comes to her conclusion of whether or not to take her Shahadah, than to pre-empt or corner her into a decision. Allow her the time and space to come to a decision.
And Allah(swt) alone knows best.