OP does not give enough info. The person seeking to enter the Catholic church would have to discuss all aspects of their marriage situation with the priest who will guide them. The answer will not be the same for everyone because there are too many variables.
Was either party baptized Catholic?
Was either party married before?
Were they otherwise free to marry?
and so forth. In the case of previous marriages, you are also talking about the circumstances of the exes.
Two non-Catholics who are otherwise free to marry are considered validly married no matter who witnessed the marriage or where. (A civil divorce does not end a valid marriage.) If either of those people wishes to become Catholic nothing has to be done to “convalidate” since it is already valid. If both are baptized the marriage is also sacramental. If one is unbaptized, and enters the Church, the marriage becomes sacramental as soon as they are baptized, but has been valid from its inception if conditions for valid marriage were present at the time of the contract: knowledge, capacity, intent and consent.