But if he is using a stock CB350 swing arm, the rim and tire may not fit diameter wise.....
Also, since stock is 18, the larger tire will reduce the effective gear ratio....Slower acceleration. The larger diameter increases the moment of inertia too, which also reduces acceleration.
Not saying don't do it, not even saying it won't fit. But you should check for fit and understand the consequences of the actions.
The ironic thing is, a 17 inch rear would allow a larger tire, for the fat tire look so many want, without clearance issues.
With any amount of work, any size rim/tire can be put on. I seen a Chevy Impala with 24 inch rims...They fit, he was driving it....doesn't make it the best idea though..