Taylor Forge Bulletin 502, Modern Flange Design, is a well known text for calculating flange stresses. There have been several threads in these forums over the last several months regarding TF Bulletin 502, its contents, availability, etc.
In 20+ years of working with pressure vessels and piping in the petroleum industry I have never heard of TF Bulletin 571 nor seen it referenced anywhere. That doesn't mean it doesn't exist, it just means I haven't heard of it. You might want to question your engineer about where he has heard of 571, what it covers, and why 502 would not be sufficient. BTW, the methods in 502 have been incorporated into the ASME Boiler & Pressure Vessel Code for flange design. I suspect that Codes from other countries have as well (AD Merkbladt, BS5500, Stoomweisen, etc.).
Good luck.