"Under Shari'a Law, if a woman converted to Islam with a marriage in Christian man, her marriage is voided if the man is not willing to convert and she can marry a Muslim man"
The marriage will be "suspended" (not voided or nullify) until the non-muslim husband converted to Islam. Correct if i'm wrong but what i know is that your "wife" must divorce her husband and enter into a iddah or period of waiting. If her husband enters in islam during the iddah, she may return to him and does not need a new contract. If her husband enters in Islam after the iddah, she may have to choose between returning to her husband or marrying another Muslim
The question is, is the husband aware of the changing of religion of his wife? was the husband got informed of the consequences if he will not "convert" to Islam like her wife? We cannot say that your wife and her husband doesn't have communication for 6 years because their child is with her husband and she is providing support for the kid. I suggest that you seek the help of a lawyer who are more familiar with Shari'a law.