No, it's ok to comment on memes that are from 6 months ago. Comment on any. Sometimes it's fun to look back at really old memes made by those you follow.
Unfortunately, the answer you get from this meme will not stop cyber bullies. Just because that person made an off comment that your friend was commenting several weeks later doesn't mean you will be able to debate that point. It is better not to engage. Don't apologize but don't interact. It's how they get their rocks off. Seriously.