Should I forbid my aunt's husband from abandoning prayer via social media, after I was previously ashamed to advise him and have repented?

Perhaps your aunt's husband prayed without your knowledge. If you are certain he did not pray, instruct him to remind her that "whoever forgets to pray an obligatory prayer should pray it" before the time expires. If the time has passed and you are certain he did not pray, perhaps he forgot, so advise him through social media or other means that "perhaps he forgot such-and-such prayer on such-and-such day." You can seek the help of a trusted elder who has influence over him to advise him.