How to deal with haphephobia in a relationship?

My suggestion : would be to talk with the person to let them know what your touch means to you, as your friend/lover, parent, child, cousin, grandchild etc. Education and then taking them to a session with a therapist and talking about it calmly lovingly will at least let the person know where you are coming from and what you expect with your touch or touch in this relationship. For most its connection. Good luck

Relationship: If the person indicates they want to be in a relationship with you that is a good start. If they have the diagnosis with an organic cause(connective tissue disease, example) then the two of you could talk with an experienced therapist about your expectations from the relationship. If it is trauma based they truly need to see a Psychologist expert in trauma-EMDR trained!