If the fingers are moved outward relative to the fastball there is some degree of supination relative to the fastball. I have know idea if that degree is enough to weaken the contribution of the muscles that handle the load on the elbow or not. For my guy, we have avoided it because I want his supinated pitches to be lower effort and I don’t want to risk confusion by adding a somewhat supinated that is thrown with high effort. However, pulling a guy off the plate with hard glove side movement would be nice.
Video is a good idea. Your fellow a lefty?