I think this was intentional to prevent backstabbing (imagine you whitelisted an "ally's" sub with navigator who can then just take your sentry's outpost with minimal damage).
The sentry is an offensive defensive unit, meaning it is viable for stopping retaliation moves (putting your sentry in the front lines to weaken enemy countermoves).
In your scenario, consider just moving your sentry, or having your friend attack in groups smaller than the enemy within the sonar range (20 should be the minimum).
Reporting from the Bridge