A 3:1 mix is only suitable for engineering bricks. The mortar will be stronger than the bricks. The bricks will be porous and accept moisture. The mortar will not. Therefore the bricks will fail before the mortar.
This is what I was afraid of and what caused the original post. Sadly, the company that did this is long gone (claimed bankruptcy about 3 years ago); I tried following up with them with no luck. I’ll just have to keep a careful eye on the chimney from here on. (A little bit of me hopes he used a 4:1 mix anyway...). Thanks for your advice.