In this situation, a pentest definitely is not on the top of priorities!

If an attacker is on the inside, your first step is to "stop the bleeding" - determining how the attacker got in so you can plug the hole. Once the company has worked through its incident response plan (assuming there is one), the thought of performing a pentest should start creeping in.

Short answer: NO!
Somebody told me, compare a Pentest with small picture.
 A picture of a wall with a whole in it. You can see the hole, but what you don`t see is, did somebody used it, whats he doing inside and did he get outside the same way? And if it is a small picture are there some more holes you do not see on that picture.

I think pen tests are most effective when an organization feels they have done everything they can to secure their environment and want to see how they measure up against an actual attack -or- there are known security deficiencies and they want verification of what the actual impact of a compromise would be. Neither of these are appropriate for the scenario you mentioned because the organization knows exactly where they stand: owned.

An odd but interesting scenario would be if they could not legitimately determine how the attack occurred. Say an attacker has installed a program that creates a basic reverse shell back to him or her, and this runs on an admin's desktop upon login. What if the actual attack, where the attacker pivoted through the network, occurred six months ago? Many organizations can't/don't retain logs past 30-90 days. All they know is there's a malicious program running on someone's desktop without a trail of information showing how it got there, and they can't determine how it could have possibly happened. It may have been has simple as social engineering, or it may have been a very intricate attack where the details are now lost. How do they validate their security posture if there are no obvious deficiencies?

Maybe a pen test would be a complimentary IH/IR activity in such a scenario. However, as everyone else has stated, procuring a pen test should never be the first thing you do upon learning you were compromised. Hopefully you've thoroughly covered the first step of incident handling and are now prepared for the rest.