What level are the PCs? Depending on their level, he could give them scrolls of sending or plane shift.
If they're really low level, and the wizard's pretty high level, Monte Cook's Complete Book of Eldritch Might has the 3rd level spell missive token. It turns a tiny item into effectively a one-shot sending back to the spellcaster -- so the wizard gives them a couple (maybe one each, just in case) of wooden talismans with missive token cast on it, and instructs them to break it when they're ready to go, and send him a message.
Then Mr. Wizard can plane shift to the plane the PCs are on, use a pre-designated bracelet of friends to call the PCs to him, and finally plane shift everyone home. Expensive way to do it, though.
That's if you want to use the rules; if you're okay with a little looseness, you can just say he gives them a wand/staff/gizmo that will plane shift them back to him when they're done. A greater word of recall token, of sorts. Call it a custom spell he knows.