Well the converse of my suggestion adapted to your particular situation would be this:

Boss: Could you do this thing for me?

Badclient: Well, as you know, I have Project X as my highest priority. I'm happy to oblige, but Jenn has dibs on my time at the moment. If you can talk to her and come to an agreement on how I should distribute my time, I'll be happy to work on your thing.

Whatever happens, your boss is on the same page as Jenn, and all you have to do is execute the agreed upon plan.