The person stepping away may miss being consulted more than being busy.

Leaving a role removes two different things, and only one of them gets discussed. The work itself is often not missed at all. What goes with it is being asked, being in the conversation, having a view that someone wanted, and that loss arrives quietly a few months later when the calls stop.

If someone you know has stepped back, ask their opinion about something real. Ask not as a courtesy but because they know things. A single genuine question can do more than an invitation to lunch.