Can I change a blocked iPhone for a gift card?

Do you mean it’s blacklisted with a carrier? If so, no. The value of that device is zero to anyone and everyone as it’s uselss. Apple will recycle it for free for you but that’s it. They will not give you any monetary trade in value for an effectively non-functional device.

The same thing goes for activation lock. If you cannot remove it, the phone has effectively zero monetary value.

Yeah, sorry your friend got ripped off, but no legitimate trade in program is going to give anyone any money for a blacklisted device. They’d be buying stolen property themselves if they did so. I suspect almost any trade in company or program won’t even take that at all, or if they do, they’ll just turn it over to the police.

Tell your "friend" to turn it over to the police. Now that they are aware that it's stolen, failure to do so constitutes a crime in most jurisdictions.

I'd suggest you ask Apple as this might be a legal problem - why is the serial number blocked? Was it stolen?

From whom online was the purchase made. If eBay or the like, he might be able to get a refund from them. It would be worth while to check.