For a large tech firm in the US as you described, it would be illegal to fire an employee due to a disability or temporary injury. That would be a form of illegal discrimination.
You may want to consult a labour-law lawyer in California if the boss actually emailed your wife telling her that he would fire her due to her temporary injury.
It appears that California has one of the strictest labour laws in the US.
You may also want to check the "employment contract" or HR handbook of your wife's company as they may have a good section stating that this company does not discriminate against disability (and other statuses and conditions such as race, gender, age, etc...)
Edit : Added more info and citations:
Thanks to user @zmike for posting the citation link, which is shown below:
The following website is run by The California Department of Fair Employment and Housing (DFEH) that is responsible for enforcing state laws that make it illegal to discriminate against a job applicant or employee because of a protected characteristic.
Here is a link to the citation on their website: https://www.dfeh.ca.gov/employment/#whoBody
On that website, please click on the section "WHAT IS PROTECTED", and you will see that disability is listed as a protected characteristic.
In addition, user @zmike also commented : On that California DFEH website's FAQ, "under "Can an employer fire an employee for being out sick?", specifies that "even temporary conditions, like a broken bone or pneumonia, qualify as disabilities when they limit a major life activity.""
(Also, thanks to user @Jim Clay for asking a good question about citations.)