I'm sure you know, the vaccine like most vaccines doesn't prevent infection. It primes the immune system to respond when infection occurs, but as with everything as complex as a mutating virus and unique biological hosts, it cannot prevent all bad disease responses. It's all a matter of probabilities. Your coworker may have been infected with a new variant before the vaccines were adjusted. It's also possible she was just one of those unfortunate folks whose immune response to the vaccine wasn't robust. Too many unknowns.
Cases like your coworkers ought to to reinforce the need to stay current with boosters and practice good judgment when in crowds, even when vaccinated.