All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
What you said to this brother is right. Actually, he is not supposed to give up his first engagement just to marry a different woman, because she trusted him and waited for him for this period hoping that he would marry her and built her future plans on that. As long as she is acceptable from the religious and moral point of view, it is not right for him to destroy all her hopes and marry someone else. On the other hand, by marrying this last lady he might not achieve his intended objectives. He could even meet a bad end. Therefore, we advise this brother to keep his promise and marry the first girl in order to guard his chastity and hers. After that, he should call to Islam the lady who wants to convert and show her its advantages. While doing that he must not let himself be pushed to talking about things other than Islam as that might have very bad consequences. If this girl becomes Muslim, he could then help her learn her religion and explain to her the kind of difficulties she might encounter from her own relatives, friends and parents if they come to know that she has become Muslim. We advise him also to help her materially as much as he can and as long as she needs that help. If then he could marry her as a second wife, he could do so. Otherwise, he could ask one of his pious friends to marry her. In addition, he should have only one objective in mind that is to please Allah. The Prophet said: “I swear by Allah! If Allah guides one single man (to Islam) by your efforts, that will be better for you than possessing a whole lot of red camels (a very expensive breed of camels).” [Al-Bukhari]
We ask Allah to grant all of us success in doing what pleases Him.
Allah knows best.