Firstly, Messenger Plus! Live is an add-on for Windows Live Messenger. This website is primarily a support forum for Messenger Plus! Live although there is a Windows Live Messenger section (which this post will have been moved to soon!)
As for your problem, has your friend checked their junk folder? Sometimes things mistakenly end up there.
It might just be best for him to download Messenger manually from
here and then to add you and hope that you are able to talk to each other. The Messenger servers do appear to be playing up a little at the moment, so that could explain why your e-mail indivitation isn't arriving.