I only like or comment on such posts when my friend is explicitly tagged in them or is the subject of the post. Otherwise it seems intrusive.
Agreed, with reservations. I would maybe do this if my Friend was the subject of the post and
I had at least some personal relationship
with the author -- like, she would recognize my name and we've met once or twice. One other exception might be for an old friend's significant other I haven't met because of time or distance, who's sharing something sweet or important about my friend (e.g., illness or an engagement or wedding). Not with a complete stranger.