If an individual has a diagnosed disease and refuses to get a clearance from his or her doctor, I am of the opinion that you must then look at your scope of practice.
The reason we as fitness professionals request that our clients get clearance from their physicians is to determine whether their condition is medically stable. Only the physician can determine that.
If the client is on any medications, only the doctor would know whether he/she takes his/her medication according to “doctor’s orders” or if they feel they can self treat or are just negligent about taking their medication.
Professionally, I would not do it because I have no clue whether the individual is medically stable as far as their disease is concerned.
Hope this helps.