Having a higher profile tyre should not cause any clearance issues as there are plenty of lowered cars driving around. Lowering reduces the tyre to arch gap in the same way that fitting bigger tyres would. However, fitting tyres with a larger rolling diameter will increase the gearing and in turn make the speedo read under.
He would be be better off keeping the 18's tucked up safely in the garage and fitting a set of 17" wheels with the more comfortable 225-45-17 tyres.