I agree with Gordon. I think your only point of argument is that coming into contact with other people is psychologically damaging and this prevents going out. However, your partner is now going outside solo.
Could the dog be considered an assistance dog? My thoughts on this are that if your partner never goes out without you or the dog then they could be considered to not be able to undertake a journey alone.
I think the dog is motivation and would not be considered a helper as such. I think the best way is to follow Gordons advice and write as much as possible about how the walking came about, its effects and side effects and maybe get some supporting evidence from GP and/or specialist(s).
