Yes! If you have a CPL, you can legally borrow a pistol in Michigan.
For most people, borrowing a pistol in Michigan would require that the pistol is obtained or transferred through a background check and registration process, either by getting a permit from your local law enforcement agency or by transferring it using a licensed firearms dealer that will conduct the same background check as the local law enforcement agency does through the NICS system.
The law that requires the pistol license and registration is MCL 28.422, Section 2. (1). Section 2. (1) of MCL 28.422 states:
(1) Except as otherwise provided in this act, a person shall not purchase, carry, possess, or transport a pistol in this state without first having obtained a license for the pistol as prescribed in this section.
There are a few exemptions to this process allowed for in Michigan law for law enforcement, military (if being used for their employment) and CPL holders.
Yes, if the borrower is a CPL holder AND the loaner legally owns the pistol. The loaner DOES NOT need to be a CPL holder.
Section 2 does not apply [if the] individual carrying, possessing, using, or transporting a pistol belonging to another individual, if the other individual’s possession of the pistol is authorized by law and the individual carrying, possessing, using, or transporting the pistol has obtained a license under section 5b to carry a concealed pistol or is exempt from licensure as provided in section 12a.
This means that if you legally own a pistol you can loan it legally to someone as long as they have a CPL in Michigan (or are exempt from needing one by having an out of state concealed pistol license).