All he needs to do is destroy Herb's beacon so they can't return.
The beacon isn't needed for their return — it just makes it easier to hit the right spot in the space-time continuum. Without it, they may be off by hundreds of miles in space or many weeks in time, but they'd be able to make it back. As Mr. Wells mentioned, there isn't a beacon where they're going.
Mr. Wells probably let Tempus in to see if he could get information on what he did with Kara. As to whether that was wise or not, well…
We'll have to wait a while to see what Tempus's evil plan for Lois and Clark is, and what happens between him and Mr. Wells.
Thank you for reading!
