What she can (must) do to prevent some people harassing her is blocking those people. Also, make sure she has set the option "Only people on my Allow list can see my status and send me messages" in her Messenger-options, in the Privacy categorie.
Also, she can log in using the status "Appear Offline". In that way, nobody can contact her, but she can initiate a chat with her contacts (eg: you).
As for your own problem: Do you use Messenger Plus!? If so, try to uninstall it and see if the problem persists. You probably need administrating powers to uninstall it though.
If you don't use Messenger Plus!, or the problem hasn't gone away when uninstalling it, the first thing todo is to report the problem to Microsoft: https://support.live.com/default.aspx?productkey=wlmessenger
However, I just noticed that some smart guy has redirected those "select your WLM version"-links to the help pages instead of the support pages. DOH! So until that is fixed, no support.
However, you can send a feedback form. In that way MS knows that yet another person has troubles with delivering messages. It is time they look into this, deeply, as this kind of thing happens way too often in my personal opinion.
So, unfortunatly the error "can't deliver the message" can be caused by a lot of things. Try to read this thread, many solutions are already given for various situations. I suggest to try them all.
If all else fails, the thing you can try is to use the web messenger: http://webmessenger.msn.com/
. Do you get the same errors there?