Can't sign with Facebook

My boyfriend uses Lightroom mobile on his ipad, I use it on my android phone. Today I signed in (via Facebook account) on his iPad to edit a photo on larger screen, then I log out and since then he can't sing in via his Facebook. When he taps "sign in with Facebook" he sees a pop up window with MY Facebook account altough I am signed out. He tried reinstalling both Facebook and Lightroom but nothing works. My Facebook account (email and password) is not saved on his iPad.

How can he fix this?