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.
I've got some info about bullying. If you go onto that person's profile page there are two choices in the upper right corner...follow or block. If you block them, their comments are hidden from your view and you aren't notified that they responded. However...and you'll be tempted...you can click on their replies to "show content" but at least it's your choice. I've had to do that to several people and it helped me feel better
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.