My father uses thunderbird for all his email. He and his wife run a business and they'd very much like to share a common contacts list.

I suppose a kind of AD/Exchange system only running with thunderbird and not costing the earth.

A third party application for contacts probably would be ideal because you'd need to constantly copy/paste addresses into thunderbird, so ideally is there something with a TB plugin or hook?

You can tell I know a great deal about all this stuff