Thank you for your question. As long as the lady is not already married, you can marry her after she has repented for the sin. You can marry her by telephone, using the usual formula for marriage and there does not need to be any witnesses. After the marriage the child still has the status of being conceived outside of wedlock. You repent for this sin in the same way that you repent for other sins, and God is the all-Merciful.
May you always be successful.