The person who cannot stay should know that their earlier help still mattered.

Leaving before something is finished carries a sting out of proportion to the facts. Whether it is a job, a caring role, or a project, the person going often discounts everything they did because they could not see it through, and the people remaining sometimes let that stand. What was given was still given. Duration is not the only measure of contribution.

If someone is stepping back, tell them plainly what their part made possible before you talk about the handover. If you are the one leaving, allow the earlier months to count, even now.