Will an iPhone 14 Pro bought from the apple store in US work in europe?

The model sold in the US has no SIM card slot so no, your Greek SIM will not work. You should check with your carrier and see if they offer service with an eSIM.

Also, did he buy a carrier locked phone? If so, you would need to contact the carrier and see if they will unlock it. Most won't until you have a t least one year service with them.

If it was bought directly from an Apple store it will not be carrier locked, but it can only be used with a carrier that supports eSIMs. In Greece the carriers that support eSIM are:

• Cosmote 
• Vodafone

As sberman pointed out, if it needs service, either in or out of warranty, it will have to be returned to someone in the US who can take it to an Apple store. Apple will not accept international shipments.

If purchased directly from Apple (Store or Online) it’s unlocked

Ref: Apple Online Purchase FAQ (excerpted screen shot)

Whether this iPhone would work in Greece or not, these facts definitely apply.

This iPhone cannot receive authorized Apple service outside the United States. Any AppleCare that may have been or may be applied in the future could not apply.