There is no problem here. Even completely legal marriages contracted among pagans can be legitimately broken with a right to remarry one party of the marriage objects to the conversion of the other and puts her (or him) away.
Baptism makes one a completely new man. The only requirement I have ever heard of a convert to Christianity is that he absolutely MUST have no more than one wife after his is baptized. If he has more than one wife, he must choose one and only one to remain his wife in the eyes of the Church. He may have to continue to support the others according to justice and civil law, but he may not have marital relations with them.
In his biography, Archbishop Lefebvre said that this was a difficulty in conversions in Africa where some men had more than one wife and often they would not accept monogamy.