Are you sure you don't mean "his online friend", or "his friend from online"? This is quite different to "meet his friend on line". To answer your question: "but we advised him not to." is the best way of expressing the sentence. There is no need to repeat the action (to meet), as it is still in the mind of the listener/reader. Repetition is redundant here. Adding "do it" is understandable, but really unnecessary.

You can say that, however this one already says the same thing and uses two words less.

because it 's imperative statement and no need to repeat the verb .