Share copyrighted stimulus material with collaborator
Non-profit research is one of the typical "fair use" exceptions to copyright law, so if it's OK for you to be using them for research, it's probably OK for you to share them with your collaborator as well.  

Every country has its own particular regulations around copyright, so it's definitely worth confirming with them. If you're just using short excerpts as user stimulus and not publicly distributing, though, that should be pretty well protected in most jurisdictions.

That bit about "not publicly distributing", however, is quite important. As such, if you want to share with your collaborator, make sure you do it with some sort of access-controlled sharing system like Google Drive, DropBox, BitBucket, or even your own FTP server rather than just posting it openly on a webpage somewhere.