alt Hacker NewsThis person has been let go and you're reaching out _to them_.
If you don't care about the person and their emotions, don't say anything. Nobody else will know otherwise (other than the person you clearly don't actually care about). Feigning care is sometimes worse than just clearly not caring at all.
Just because you have a limit to how much you're willing to care about somebody doesn't mean that you are feigning the care that you do give.
Most people have reasonable expectations about how much care they can expect from some random colleague from a past project.