The Allegro design file is a proprietary binary database. All methods that I have ever seen to get data out of the database for a translation require having Allegro installed. Some methods use the extracta utility from Cadence. That utility is part of the Allegro installation, but does not require a license to run.
Since you need an installation of Allegro anyway, it seems that your easiest path is to go back to the person who provided the .BRD file, and ask them to export the ODB++ data.
Mentor already offers translators for most raw board files that are run through the main HyperLynx "New Board" PCB Translator. Allegro is one of these. I had better success running the command line from the .ERR file myself manually and then opening the .HYP file.
If you need to run it standalone, they also support downloading the tools separately, but I got licensing errors when trying this, so YMMV.