If you'e using air or nitrox, you're unlikely to get any benefit from Argon...
The first stage can be any conventional one, but it needs either a DV or an OPV (Over-pressure valve) in case the reg decides to freeflow.
Remember to mark the cylinder very clearly as being non-breathable air, there's been at least one death from somebody deciding to go pootling around the swimming pool using daddy's little scuba tank...
You might want to look into something like the Xerotherm for her instead - I bought my mother some of the Xerotherm socks, and they keep her feet warm. There can be no greater endorsement!
