what are the best massage therapy techniques to use on a client with RA

Whatever you choose to do-be very very gentle. No deep tissue stuff for sure.

I would recommend you speak to a registered massage therapist about this as they are more likely to be familiar with the various techniques used in massage therapy.  Massage can be beneficial for people who have RA, but aside from recommending that you err on the side of being gentle for several occasions of massage to discover where the person's pain limits are, those of us who answer questions here can't really give you any kind of expert advice in this area, as I have yet to see anyone who is a massage therapist on the site, so I would consult someone who has studied massage therapy.