What language(s) do they speak? Are they able to read enough basic French or English words to navigate the "connecting flights" path at CDG? That's really the biggest area where they *might* need help, although most airports will nudge you in the right direction if you just show an airport employee your Montreal boarding pass.

I don't think professional greeters are a big need here - and don't even know if they could get into the international connections area of CDG to begin with. Biggest thing is to simply make sure they have a comfortably long layover at CDG, and have a plan in mind for what to do in the case of delays/cancellations.

Once on the ground in Montreal, everybody's pretty much following a single flow until they exit customs and their grandkids can collect them.