We offered an arthritis exercise class open to the public that met two days a week. The therapist proctoring the class decided to not renew his certification. The facility would like to continue to offer this program to the community, this is a free class. We would like to get another therapist involved to continue.
As Karin suggested, I think the first thing you might want to do is determine what certification your therapist had. In addition to arthritis specific training like she linked you to, you might also want your next person to have a group exercise certification such as AFAA primary group exercise or ACE primary group exercise.
It typically takes someone who is new to fitness about 3-4 months to study for a group-ex certification, although a highly motivated person or someone with fitness / anatomy / experience could do it much quicker.