Dell Inspiron 3650 SSD

My friend bought a Dell Inspiron 3650 desktop. Next to the HDD inside (3.5 inches) he wants a 120GB SSD for it to be the system partition, and the HDD the backup storage.
In the specifications of the machine I found "two 2.5-inch drive or a 3.5-inch drive." at the hard disk drive section.
Please help me because if I understood correctly from the documentation, what we want is not feasible, because the SSD is 2.5-inches and the hard drive is 3.5 inches.
In addition, studying the connectors on the system board from the SSD, SATA connector can only be considered?
Thanks in advance for your kind answer.