When I see a thread like this, the first question - what is the OP hoping to achieve? Sometimes the question asked is not the real question.

A "Bitcoin User" doesn't have a public key, a Bitcoin address has a public key. The public key is only published when a transaction spends funds previously sent to that exact address.

Only after an address has spent money can the public key be recovered from a transaction stored in the blockchain. By recommendation, addresses are often never reused. Nothing about the address or its public key informs you about other addresses the user may have or use.