If someone has social security retirement can they apply for disability.

Yes, someone receiving retirement can apply for SSDI benefits if they elected to take early retirement and are receiving a reduced amount. If they retired at full retirement age then their SSDI benefit would be the same and there is no need to apply. 

They would need to prove that they are medically disabled and unable to work at SGA. (Some people who take early retirement benefits are ready to stop working but don’t meet the definition of disability).