HiI think it's great that you're making an active effort to involve the deaf community in your productions! I cannot claim to be an expert on the legal aspects of hiring an interpreter and who is responsible for that, my apologies, but it sounds like you have several experts here!
I wanted to mention as you're so close to Mishawaka/South Bend, you might want to contact Bethel College (incidentally my undergrad alma mater). They have excellent Sign Language program and their students and faculty regularly interpreted for our theatre productions. If you are looking to hire someone close by you could inquire. I don't currently know their policies on freelance work for students because of licensing, but it might be worth a shot? And if not, they would probably have resources for local interpreters.
This might be in another direction entirely, but if your student doesn't already know about the Deaf West Theatre Company (http://www.deafwest.org/), I'd encourage them to check it out! Their production of Big River was mesmerizing! Also, my college roommate who went through the previously mentioned program now works in the D.C. area as an interpreter: she and her husband (who is Deaf/Hard of Hearing) introduced me to the Synetic Theatre (http://synetictheater.org/) which produces "Silent Shakespeare" among other productions geared towards the deaf community, among others. A rather interesting translation of text into other mediums of art!
Best of luck!