I wanna know too
I can only speak from a friend's experience-- when he got beat up by loansharks and rang the police they told him it was a civil matter and that they couldn't do anything. They pointed out that my friend borrowed from a sarakin, so it was all legal and that he should be responsible since he knew what he signed up for. It was a goddamn mess, but me and other foreign co-workers helped him out of it. This happened way back in 2012 though, so not sure if Japan's police stance has changed. More to answer your question though: if they're not yamikin, the police probably wouldn't do anything.
Eww... I'm glad you saved your friend tho!
Like...what would happen if the authories were called? Would the loan shark even get in trouble?