The best thing to do is to ask your friend directly - via private message or their visitor's messages, if your friend has one of those set up. If not, you can also email, if that's set up under their contacts, or you can tag the person in message within that forum.

However, if it's a group thing that you need to get access to, you can also add membership to that group by going clicking on "My Profile" icon at the upper left, then on the left side you will find "Networking" and under that "Social Groups," - click on that, then you can find your friend's group by name. You can try to also find the post you wanted to respond to, but you'd have to first add yourself to that group in order to respond.

You can PM me and I can help you through that process - step by step.

If the person has ignored you, then you can see your friend's name but not what that person posted.