Here's my suggestion: have your boss talk to Jenn and make her understand that you'll be spending the next day working on the boss' request. Any interruptions would have to go through the Boss, so that all parties are aligned.
This seems like a communication problem between the managers. (I can speak from experience, since I messed this up once when I reallocated our engineers' time without fully being on the same page as the engineering manager)
Ideally that should work but projectX is what the whole company is working on and key pieces of it are being tested throughout the day so it's impractical to have a cut off time.
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.
This seems like a communication problem between the managers. (I can speak from experience, since I messed this up once when I reallocated our engineers' time without fully being on the same page as the engineering manager)