Bib fortune has 3 resources but is destroyed if he has 4 focus.

To my understanding a unit cannot take focus once it has focus ie exhausted.

MY friend believes that he is able to use a Focus combat icon during a battle to add the 4th focus on bib even though bib is exhausted with 3 focus on him.

So is he able to focus bib even though page 10 states that you cant focus exhausted units?

Bib fortune has 3 resources but is destroyed if he has 4 focus.

To my understanding a unit cannot take focus once it has focus ie exhausted.

MY friend believes that he is able to use a Focus combat icon during a battle to add the 4th focus on bib even though bib is exhausted with 3 focus on him.

So is he able to focus bib even though page 10 states that you cant focus exhausted units?

Units cannot voluntarily focus if they are already exhausted (have focus tokens).

However, there are plenty of ways for the LS to add a focus token to him, like Jedi Mind Trick and tactics.

There is a difference between "focusing", which usually produces some result like generating resources or triggering an effect, and "placing a focus token", which just adds a focus token to a card. In the case you mention, the Tactics icon allows that striking unit to place a focus token on any enemy unit (not just those participating in the engagement), so yes, he can add a 4th token to Bib and destroy him.

According to your interpretation, his ability could never, ever, ever activate, since there is no way beyond other card effects and tactics to get a 4th focus token on him. That's usually a good indication that it might be wrong.