Yes, it's an absolute dick move. If it's that bad, accelerate his cliff and let him leave early.
Had a situation where I was let go 2 months before my cliff in an advising/part-time situation, and while I won't say anything negative about the company and most of my colleagues, I was honest in respect to the exec teams behavior. Zero of the 4 applicants who I relayed that to accepted their offers. Was my small equity stake worth (potentially, may have had other reasons) 4 hires?
Had a situation where I was let go 2 months before my cliff in an advising/part-time situation, and while I won't say anything negative about the company and most of my colleagues, I was honest in respect to the exec teams behavior. Zero of the 4 applicants who I relayed that to accepted their offers. Was my small equity stake worth (potentially, may have had other reasons) 4 hires?