If they had kosher witnesses, as is the case with many frei Israelis and sefardim, then there’s no need for a new kidushin/chupah/kesuvah. If there weren’t any kosher witnesses, such as a typical reform or conservative marriage, then they would need to do everything over again.
That’s what most poskim say. Rav Henkin held that at some point kosher witnesses probably saw them going into the husband’s house, etc..which is enough to make a kiddushin. It could be he said it lechumra to necessitate a get, but not that they don’t need to redo their marriage. Whereas most poskim held that a standard frei ceremony wouldn’t require a get, and the wife can remarry at will. I don’t know what the accepted practice is among rabbonim in that case, but baalei teshuva routinely get remarried al pi halacha
As for a chiyuv to divorce if one spouse becomes frum; this is very, very complicated and each case is different. Sometimes a frei person will agree to keeping a kosher kitchen and adhering to taharas mishpacha, while not being personally observant otherwise. This happens quite often.