One suggestion, perhaps the most obvious: identify the likely pay/compensation of the person who will replace them, and then pay them the difference (or as much of it as you can) in severance over and above what you're required to offer.
Second suggestion: help find this person a new job at the kind of pay they should actually have been earning for the quality of work they were doing.
But don't feel guilt: it's more like a moral obligation. It's a good thing, not a negative emotion. It means you're thinking like a complete person in a world where it is common to pretend that isn't necessary.