>Upon his return, he was given a warning for each day missed.

This is called "retaliation".

(c) Earned sick time shall be provided by an employer for an employee to:

(2) care for the employee's own physical or mental illness, injury, or medical condition that requires home care, professional medical diagnosis or care, or preventative medical care; or

(h) It shall be unlawful for any employer to interfere with, restrain, or deny the exercise of, or the attempt to exercise, any right provided under or in connection with this section, including, but not limited to, by using the taking of earned sick time under this section as a negative factor in any employment action such as evaluation, promotion, disciplinary action or termination, or otherwise subjecting an employee to discipline for the use of earned sick time under this section.

Source

Get a lawyer. Do not take this issue directly with the employer - without having a lawyer to back you up first. Tell your father to try to avoid meetings with his bosses and HR - get everything in E-mail. If he has to go to a meeting, take notes and put them in an E-mail to the attendees and ask them to confirm your notes are correct. Don't sign anything without a lawyer present.

If your father has his most recent performance review prior to the injury, that would also help.