Following My Fathers Brain Surgery, Hr Is Harassing Him Claiming That If He Does Not Return To Work Then His Job Will Be At Risk.

Just to give some background, father works a local government job and is extremely good at it always going above and beyond. He is loved by his co-workers and typically leads job related extracurricular events.

However my father recently had emergency brain surgery to remove a tumor that was discovered following an unexpected seizure. While he is fortunately recovering smoothly, he is unable to drive for about another month and is unable to do his job. Recently HR has sent him a letter basically saying "Come back to work or else your job is at risk". Are they able to do this? He has not been cleared by his doctors to work/drive yet, but he is fearful that they are going to fire him. In total my Dad has been out of work for about 3 months but has been doing everything he can to help remotely. He responds to emails, answers his coworkers questions, etc.

My siblings and I theorize that they are trying to fill his position because nothing is getting done while he is gone or they are trying to fill his position with someone's family member. My Dad truly goes above and beyond when he works, he comes in early, he leaves late, he constantly is studying to get new certification, etc.

If they are to fire him do we have a case for an unlawful termination?

Tl;dr: Dad had brain surgery, HR Said return or else. Doctors havent cleared him yet. Can they legally fire him? Can they deny ADA requests as a result of his surgery?

Edit: This is in Oregon and I believe my father is approaching the end of his 12 weeks under FMLA. If he requests ADA accommodations and they deny him is this legal?