Suboxone for Pain Control?

My sister was diagnosed with stage 4 metastatic breast cancer in April 2019. The cancer is aggressive and has spread just about every place imaginable. The pain management doctor who happens to be the chief doctor of this huge practice has prescribed 32 mgs of suboxone daily and 3 600 my gabapentin daily. The gabapentin does eleviate some pain but she's still suffering. As a nurse myself I have never heard of suboxone for pain relief. It's predominantly used for management therapy for opiate addiction. Anyone else prescribed this?