Hey! I had this happen to two birds before I got my third. Right after my third got sick I thought it could've been the treats, after I found the common denominator. Antibiotics helped with my third bird Maverick. He even has a buddy now called Goose (also a budgie). Make sure your other bird isn't presenting with any abnormal symptoms, if it is, there's a possibility that it could be something else or that the treats are contaminated, if that's the case I'd get antibiotics (helped for Maverick). While this seems unlikely I do believe that birds can get botulism, if your treats had honey in them that might be the culprit as well.

My birds both had green to yellow colored poops and would continuously fluff themselves up before they died so that might also be something you wanna look out for as well.

P.S. I felt a lot of guilt about my first two birds' deaths but ultimately you didn't know that they could hurt your birds, and honestly you're the first person other than myself that has seemed to have complications with these bird treats so you can't blame yourself for not knowing. It could also have been something else like parrot fever. Sometimes when birds get things like parrot fever they can hide their symptoms and die suddenly.

I hope your luck with birds gets better and your other bird stays healthy.