All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The evidence which we mentioned in the Fatwa you referred to is enough and it proves clearly, that it is permissible for two men to enter upon a woman who is not a Mahram to them; this number [two] is enough for negating the forbidden seclusion with her.
Therefore, it is permissible for a new Muslim woman to sit with her (to-be) fiancé in the presence of her Muslim guardian even if he is not her Mahram.
However, we believe that there is no benefit in this meeting, because it is not permissible for her guardian who is not her Mahram to look at her in the same manner her fiancé is permitted to look at. In this case, it is an obligation on her to bring a Mahram with her and it is not a condition for him to be a Muslim; being a Muslim is only a condition for being a guardian in the marriage contract.
In case she does not have a guardian or it is not possible to bring him for any reasons, then she may meet the fiancé in the presence of another woman in order to negate the forbidden seclusion.