Absolutely not.

Your colleague's career decisions are none of your business. Their relationship with management is none of your business.

Beyond that, the relationship between employee and employer should be professional. Professionals understand that people--even important people--sometimes leave, and they plan for that. Thinking you need to look out for the company is a misunderstanding of how the professional world works. If the company wanted to ensure a certain term of employment or provide some fallback in the event of someone wanting to leave, they would have written that into the contract and paid accordingly. They didn't, so they've knowingly accepted the risk of this person leaving.