Hi cobinson,
Welcome to Vista Forums!
They are different Messenger versions (that much is true), but there's no reason in the world why the two cannot connect because of that. In fact, there are even newer versions of Messenger (e.g., Windows Live Messenger 2011 which is what I use and love
Windows Live Messenger 2011) and they will
all connect with each other regardless of the operating system. The problem is caused by something else. Perhaps he has her blocked for some reason (or by accident) - check that to make sure. Can he connect to her Messenger (in other words, does the problem occur both ways or only one way)? By "connect to" I'm assuming you mean communicate with and exchange files and information and "do what Messenger does" and not some form of remote access or remote desktop control. If I'm wrong, please clarify exactly what you mean.
I'm not sure how tied you each are to your versions of Messenger, but perhaps you should both consider upgrading to the above product and not only get a newer and better Messenger, but possibly solve the problem at the same time. You can always uninstall the new version if you don't like it.
If this doesn't solve the problem, then his configuration is blocking her (or it may be due to firewall settings or security software settings - again something blocking the connection). Try disabling them and see if the connection works then (but turn them back on immediately after the test no matter the result). Does this problem happen with anyone else - certainly you know other people using Vista you can try connecting to to verify it works? With Messenger, you're dealing with people with all types of Operating Systems and versions of Messenger - and if this is the only connection with a problem then the issue is with the configurations. It is NOT because of the different versions of Windows or the different versions of Messenger.
I hope this helps. If the problem persists, post back with details of what you tried and what happened when you did.
Good luck!