That sounds very open-ended. Material witness holds are indefinite. He does not have to "know" the defendants in the case, if he saw something, they are keeping him for testimony as to what he saw, where he saw it and when. The troubling aspect of your question is the immigration issue which sounds like he might be deported upon release.
https://www.inmateaid.com/ask-the-inmate/how-long-are-inmates-detained-on-a-material-witness-hold#answer
Accepted Answer Date Created: February 09,2016