Many programs accept a points transfer as activity to keep a dormant account alive after the 3-year limit, so if Aeroplan does this, he can always move some of his Amex MRs into his AE account. However, there is no way to move AE miles into his MR account. [Well, there is points.com, but the transfer rates and costs are outrageous.]

BTW, technically, AE miles expire at the end of every calendar year, but AC renews the program each year thus extending the validity of our miles for another year [plus another 11-1/2 months given to use them].