Male friend, we go out on week ends, FORCED to loan his car to adult child whose car will be in the shop for several days. The way I look at it, it is not my responsibility to then loan friend my car so we can go out. What suggestions do you have???

First ... I highly doubt he was forced to loan his car to anyone.
All he had to say was: "No".
If he needed an excuse: "Sorry, my insurance doesn't cover other people driving it."

Second ... No.  You are under no obligation to loan him your vehicle.  If you both still want to go out + he doesn't have a vehicle + you have a vehicle = you drive him in your vehicle.  YOU drive.  HE is passenger.

Oh no! Adult Child! Man Baby's arch nemesis.