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...
You should create multisig transactions. Multisignature (multisig) refers to requiring more than one key to authorize a Bitcoin transaction. It is generally used to divide up responsibility for possession of bitcoins.
Standard transactions on the Bitcoin network could be called “single-signature transactions,” because transfers require only one signature — from the owner of the private key associated with the Bitcoin address. However, the Bitcoin network supports much more complicated transactions that require the signatures of multiple people before the funds can be transferred. These are often referred to as M-of-N transactions. The idea is that Bitcoins become “encumbered” by providing addresses of multiple parties, thus requiring cooperation of those parties in order to do anything with them. These parties can be people, institutions or programmed scripts.