Possible workers comp fraud

Hi new here,
I run a couple restaurants and recently brought on a prep cook to cut meat for a trial period of one week. I told the employee we would give him a shot and see how he does. He works that Tuesday, he does ok, I tell him we don't need him Wednesday and to come Thursday. That Thursday he gets hurt at his other job. Some type of back injury. His full time job is evidently supposed to pay for his workers comp of his part time even though he only worked one day with us on a trial period? He wants me to put in writing that we were going to schedule shifts for him beyond the date of his injury, which was of no guarantee. He is suing workers comp or possibly his company for not paying his shifts at our place of employment. What I want to know is if I should report him to workers comp fraud, or if I should do nothing, or write a letter with the truth that all his shifts were discretionary and we never promised or finalized any shifts and it was made clear that his work was trial only. He was paid for his single shift and we have not scheduled him since.
Thank you for the helps guys. State is MN