If he went through the process of registering it with the ATF, and paying the tax on creation it might be able to be gifted, that area is still grey, hell at that point he might even be able to sell it. But with the ATF grey areas should be avoided at all costs, or at the minimum get a letter from them on how they feel about it this week.
As an individual , the intent can not be to sell. If you built one decided you hated the color or whatever you can sell it or gift it. . You will have to meet atf marking requirements . You can use the serial because it is a different manufacturer. If you build a bunch and start selling them. you will need an sot. Transfer rules apply . the buyer or gift receiver must be legal to posess, for personal use you can build as many as you want.