All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
It is not permissible to like or show approval for posts and videos that contain sinful content, such as women displaying themselves immodestly, music, and other prohibited elements, because expressing liking for such content while it contains these wrong matters implies a degree of approval and contributes to its spread and promotion.
You should also advise your friend who appears immodestly in pictures, as giving sincere advice is part of our religion. Furthermore, there’s no harm in congratulating her on her success, and you can combine giving her advice with offering your congratulations.
Defending Islam is a duty upon every Muslim according to their ability. However, trying to monitor and respond to everything that is said or posted on social media is beyond one’s ability and is not something the Shariah requires, as it would be extremely difficult.
Also, defending Islam against criticism requires sound Islamic knowledge. If someone lacks proper knowledge, they should not expose themselves to doubts and misconceptions, as they may end up being influenced by them and unable to refute them, which could lead the person astray.
The Shariah has warned us against approaching sources of doubt, disbelief, and the words of misguided people. In the Hadith reported by Imam Ahmad in his Musnad and by Abu Dawood
in his Sunan, ʿImrān ibn Ḥuṣayn
said that the Messenger of Allah, sallallaahu 'alayhi wa sallam, said: “Whoever hears of the Dajjāl, let him stay away from him. For by Allah, a man may come to him thinking he is a believer, but will end up following him because of the confused ideas (doubts and misconceptions) he spreads.”
Ibn al-Qayyim said in ʿUddat as-Ṣābirīn: “Nothing helps a person stay away from evil more than distancing themselves from its causes and places.” [End quote]
Lastly, as for your question about what to do with your old comments, if you mean the ones you wrote yourself and which involve anything that goes against Islamic teachings, such as expressing liking for impermissible content, then you should delete or remove them if you’re able to. If you can’t, then know that Allah does not burden a soul beyond its capacity.
Allah knows best.