There are no Super Redhawks in 45 colt, so he either has a Super Redhawk in 454 casull which has been milled for moon clips by a custom smith, or more likely, he has a regular Redhawk in 45colt/acp, which was factory milled for clips.
True SRH's have Carpenter steel cylinders which are designed to withstand the 65,000psi 454casull. The 45 colt Redhawks, alternatively, sport 410 or 416 stainless cylinders, which are designed to withstand 21,000psi 45acp loads - BUT have been proven to tolerate loads approaching 50,000psi. Considering the 40,000psi speed limit of the 45win mag, you'll be grossly out of the intended specification, but well within the proven structural tolerance of the Redhawk cylinder.
The risk in using 45win mag in a Redhawk, albeit apparently exceptionally small, is fully assumed by your friend. Personally, I'd just load hot Tier 3 45 colt loads and live happy, since Win Mag brass isn'f always easy to find.