Double Click Side Button Disappeared?

Please follow this path,

iPhone > Settings > Accessibility > AssistiveTouch — disable, if it is disabled, momentarily enable and then disable again. 

Scroll down to bottom of AssistiveTouch screen > Confirm with AssistiveTouch > disable

After confirming above, please do a forced restart following the instructions in the Apple Support article linked below. If you’re unfamiliar with the procedure, follow the step by step instructions in the article. 

Force restart iPhone - Apple Support

Look for side button switch.

You don’t have any payment methods in your Wallet app.

Like Jeff said, you have nothing in your Wallet.

What country is your friend in? What Region is his iPhone set to?

What iPhone and iOS version?

Does the iPhone have all of the prerequisites for using Apple Pay?

Is the iPhone part of an Apple Family sharing group? Is the device a managed device (business, military, school etc.)?

It is on my iPhone 16 Pro Max. Have you tried double-clicking the side button to activate Apple Pay?

I'm sorry, but I'm out of thoughts.