It’s not your place to tell the boss. What if they don’t get the new job and now the work dynamic has changed due to your notification. You don’t even... know if they want the new job. I interviewed all the time with competitor companies as a designer to see what they offer so I can ask my home base employer for reasonable raises. Stay in your lane, it isn’t your business or place to pass that information forward. more

You need wisdom here.
Tell your boss the truth

First find out from the designer why he is leaving. If it is about salary and condition of service, then let the boss know and match it or even give... a better offer as your new project was taken because if his expertise.
Thank you.  more

You don't
Never do anything that will come between a person and his means of income.
Imagine they end up not hiring him
Telling your boss will... result in him getting replaced either way
It is easier to get a new designer than for a designer to get a new organization more