You file the chrome off first to the depth of the fitting plus a little more, so the solder can adhere to the copper.
Wish i had known that back in January! haha
That job does not look like a pro did it. Makes my work like i've been doing it for 10 years, hope it gets sorted out. I would tell him I want it sorted once and for all within a week or he will not be receiving payment. If he needs to speak to the manufacturers about fittings tell him that's his problem it's not stopping him from sorting your house out.