imported non spendable btc
Impossible unless you have the private key belonging to the adress.

Your friend didn't import a few "wallets", he imported a few adresses, which makes his wallet a "Watch-Only wallet" (for the adresses that were imported.)


  Is there any way i can retrieve them via finding the private keys for them to make the btc coins spendable again?


Again, if you only have a public key, master public key, or adress format, the answer is no. 

If you have a seed / private key from which the (imported) address xpub/public key was originally derived from, then yes, you could recover his funds.

Edit: I feel like we're missing a whole lot of context here: Did your friend buy these "wallets"? If so- he most likely got scammed.