The advice still stands. It's not impossible to do, but there are permits he really should get or be sure that he has if he's going to do this.
Even if it's part time, he still winds up manufacturing parts for replica guns and selling them.
If he only makes one or two bits and pieces, then it probably wont matter.
If the item is considered Prohibited, then he could be in deep shit unless he has the right paperwork. I notice you changed the wording of your post, but you just placed this guy into a public forum that anyone can read, and the picture still is titled 'silencer'... is that fine with him?