Unlike one of the comments, you cannot download keys from Apple - they exist only on its computer. You can only download provisioning profiles from Apple. Therefore, if you do not want to cancel the existing distribution keys, it will need to export them from your computer.
He can do this by going to Keychain Access → My Certificates and looking for an entry called "iPhone Distribution: My Company Name". Then right-click it and select "Export ...".
Then he can send you this file, which you can double-click to import into your own keychain.
And it’s good to have several keys on your computer - this should not affect your personal account.
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