It sounds to me, like they just want to get rid of your father, maybe it's his age and they want to bring in someone younger.
Employers will use many tricks when they want to be rid of an employee, sometimes fabricating things and finding fault where there is none.
This has happened on two ocassions where I work, one employee had worked for the company for 10 years, but the new GM, wanted to bring in someone from his previous company, but in order to create a role, someone had to go.
They ended up putting a spy in and taking photos, trying to implicate her and find faults. They gave her a verbal and written warning and it seemed like only a matter of time before they gave her the push. She ended up leaving because the victimisation was becoming too much for her.
Your father should not take this kind of rubbish lying down. It can't be right or fair that your father's employers are taking his colleague 's word as the gospel. Sounds like he has grounds to sue for harassment, bullying and possible ageism.