Long story short. I've been a developing a game for a ~2 years now and release is coming. In the middle of the development I was helped by my friend (she is a designer). She helped me to create most of game's UI and some of the arts. This is about 70% of all game's artwork. Everything else is on me.
We never agreed in writing or verbally on any reward and it was a friendship team up. She wanted to gain some experience and I needed a designer.
Right now she is not in the team for a personal reasons. We worked around 3 months in total.
All assets done under my tight control. The game is set to be released on Apple AppStore. It will not be free. Plus I own all the source files as she gave it to me.
The question is: do I have to retrieve any copyrights from her? If yes then how? What is the process of retrieving rights if there is a demand for one?
I wish I could use all the assets exclusively and I want to be safe from just-in-case situations. The thing is overcomplicated with that she is in the other country right now.