How to obtain the files created by previous university researchers, both legally and physically?
Talk to an attorney or post your question here: https://law.stackexchange.com/
The files could still exist in a Univeristy network but you have not provided enough information to know for sure. Maybe someone here can help: https://security.stackexchange.com/ 
I would not keep sensitive personal information on a work computer and would have no grounds to compain if others later gained access to that data.
I see no ethical problem with recovering any and all files on the University netowork. Ethics regarding file recover the personal computer of the RA would depend on the ability to first obtain a court order to search it or demand the data in question is turned over in which case I see no ethical problem.