It is quite simple: 

The horse as a sentient being.

The client as an equal team member, along with the Mental Health Professional and the Equine Behavioral Specialist. 

All of whom in a square - each one taking responsbility for himself. 

That is the essence of the Quadrant approach.

The horse is not a therapist, tool or attachment substitute.

The client is not broken, damaged or less.

Neither one in the team is taking responsibility for the other - and all of them are working together to assist the person asking for help.