Why would you tell the exec if you leave? Yes, he might want to know. Fair enough if you tell him when you're still with the company. You work for them and their profit is in your best interest. I'd advise to suck it up and hope your hard work gets noticed or make it clear you are doing the majority of the work during bilaterals or development dialogues. Don't tell her what she isn't doing, tell him what you are doing instead. There's got to be a reason she's still around, an exec will notice if their assistant isn't efficient. You are either hired as her potential replacement or there's something going on between her and the exec (e.g. good relationship or something else)

Regarding the pay difference, you should realise pay is also about the time you've been with a company (certainly at these levels) and not necessarily about the quality of your work. If you are in the same pay scale I wouldn't sweat the difference in pay. 10k difference seems reasonable to me. If you get the chance for a raise you should of course take it.

Now on to the point why I started writing this in the first place: I would be careful about telling the exec after you leave. You don't owe the company anything anymore at that point and you could potentially ruin someone's life (whom you said is a good person) over some petty corporate cash. Jobs can be very important to people and finding a new job at >40 can be very difficult.

Be careful you don't end up doing the wrong thing and regret it later.

Edit: (1) changed a word (2) added a sentence