My friend when gets stressed out or extremely sad , blood comes out of the mouth, what is the reason and cure for this ?

Unlikely: Blood from mouth or gums should not be caused by stress. Unless there was inadvertent trauma (biting tongue or cheek). See your doctor and have a CBC (complete blood count) done to rule out more serious causes such as low platelets. Make sure you tell your doctor about all medications you are taking.

Biting himself?: Is your boyfriend possibly biting or otherwise injuring the inside of his mouth, making it bleed? Is he having seizures? Has he had his blood count checked recently? For sure it sounds like he needs help for his stress and extreme sadness, which his doctor can provide himself/herself or refer him for. Psychiatrists and psychotherapists can be very helpful with that part.