Personally, I would simply ask why it wasn't given at the time. It's not your fault it wasn't done properly, it sounds like there are many issues with this "rescue" and I would cut ties as fast as possible.
I have nothing against rescues, I've dealt with some great ones through my parents who foster for a rescue and another which my sister adopted her little guy from but some are...less then reputable.
Jasmine - Female Miniature Poodle - born Aug 15, 2010
Loker Delgado Von Stalworth - Male GSD - born Jan 26, 2012
Koda & Zazu - 5 year old male cats
Alex - Male Cocker Spaniel (rescue) - RIP Cuddlebug 2007-2010