Knowing that guidance comes from God and God alone.
Making sure that as a Muslim, you do not act in a way that will LOCK the door to this person's Islam. Instead, with your actions, words, and behavior, allow the person to see the beauty of Islam on a consistent basis.
If the person is constantly speaking about their apostasy and making a spectacle of it, protect your children and younger family members from their discourse.
Clearly, something - emotional, intellectual, or spiritual has occurred that has caused this person to leave Islam. It is entirely possible, they didn't even understand Islam properly in the first place.
The job of the everyday Muslim is not to act as the Qadi, Amir-ul-Mu'mineen, or authority. The job of the every Muslim should be to exhibit Prophetic character and manners when dealing with anyone.
As long as the person is alive, they may come back to Islam. Allow that by not reacting with harshness. If you are friends with this person, see if you can answer their questions or take them to someone who can. If not, leave them be.
Allah guides whom He wills, and Allah knows best.