If you're trying to post through a web browser (rather than via a dedicated mobile app), it might be worth clearing its cache (and/or trying a different browser). That sometimes fixes such problems.
If you need instructions, please say which browser you're using.
so you go on your mates profile and the blue message button on the right is disabled? All your friends? cos if it's just one then they've changed their settings on purpose or by mistake to stop people posting on their wall.