How can you share a plaintext bitcoin address private key with a friend and ensure they won’t spend your bitcoin? How can you share a plaintext bitcoin address private key with a friend and ensure they won’t spend your bitcoin?
That's not possible. If your friend has the private key, he has everything he needs to spend all the bitcoins of that address.

What you could do: Use the private key (before he uses it) and send the coins to a new wallet and don't give him the new private key, seed...