My character has PTSD, but doesn't have access to a therapist. All they have is a reluctant adult, so what kind of therapy could they do?
They could listen to and be supportive of your character, but you can’t call what they’re doing “therapy”. Therapy actually requires a lot of training and a license to perform. When dealing with severe mental illnesses like PTSD, it is hard to make progress, and very, very easy to make things worse. This is akin to asking how someone without access to a dentist could treat cavities.