Ok, definitely visit
www.rda.org.uk, and direct them there too as it'll give a lot of information.
I don't know a huge amount about setting up an RDA group, their best bet would be to phone up the RDA and speak to them about it, or maybe find the nearest group to them and visit. It might be that an exisiting group may be able to set up riding sessions on their land. My old group used to have 3 different places they rode at, which were either existing riding schools or private livery yards. The owner of the RS/livery yard would let them use the school and then either borrow horses, or in the case of the private yard, the group kept 2 horses on livery at the yard and paid a minimal livery fee to the owner of the yard.
An RDA group basically consists of a commitee - a chairperson, treasurer, secretary and other trustees, who together run the group. In order to set up an RDA group your friend would have to apply to the RDA to set up a new group, find people to be on the commitee, and then have inspections of the yard to make sure it was suitable for RDA. There are a lot of rules and regulations with running an RDA group - there's a members handbook (we used to refer to it as "The Bible"!) which explains all the rules, i.e. manual handling, safety, risk assessments, what tack can be used etc. etc. Basically the RDA have to ensure that the yard is safe, the horses are safe, the people helping are safe, there's suitable equipment etc. It's a lot of work, but is definitely worth it in the end!