Since when does a great learning experience equate to thrown her to the sharks? If I'm providing you with a great learning experience, then I'm not throwing you to the sharks. He could have mentored her and start transitioning her away from being a junior developer.
Dude, you're kinda rude. You try mentoring a junior FE dev into a full stack dev in 2 months and capable to build an app end-2-end with devops and everything.
We do work together on the app so she's learning stuff.
I suppose that's asking for too much.