He needs to formalise the claim quickly as paperwork, was he off work? was it reported as a RIDOR?
The contractor and client (your mates employer) are both liable, there should have been a “construction phase plan” in place for the work. I doubt this exists and you ideally need to go legal before they write it retrospectively
Overall. It’s not your mates responsibility to avoid hazards he isn’t aware of it’s the contractor/ client. They have already either failed to write or adhere to a competent construction phase plan under CDM2015 for the works. They will both have insurance for this.
BTW, I’ve done daft things to myself at work and didn’t even think of claiming. this is different in scale and responsibility