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 'alyhi wa sallam, is His slave and Messenger.
It is forbidden for any Muslim woman to establish any relationship with a non-Mahram (marriageable) man even if he promises to marry her. The promise and agreement for marriage do not render permissible what Allaah has forbidden, like staying alone with her, looking at her and the like, unless he has actually conducted the marriage contract with her.
It may be that there is a promise and an agreement for marriage but in the end, the marriage may not take place. Indeed, a happy marriage is the one which begins with the obedience of Allaah and not that which begins with disobeying Him.
Finally, you should know that it is lawful for a man who proposes to marry a woman to look at her once or twice in order to get to know her and to ensure his desire to marry her. But he remains a non-Mahram to her like all other marriageable men until he conducts the marriage contract with her.
To conclude, it should be noted that it is dangerous to be lenient in such matters, as it is the woman who will be the great loser in this regard.
Allaah Knows best.