How do I get an employee's partner to stop getting involved in company business matters?

I think you’ve said it all, already.
Clearly you are talking to a brick wall, soooo….just go completely mute. I mean total, pin dropping, silent, when in this persons company.
Go silent and walk away.

Actions speak louder than words.
If you feel the need to utter any more words towards this jackass, become a broken record.
” I have told you that your behavior is inappropriate, I am leaving now.” Then just freaking walk away!

There is nothing you can do except make an exit. Good luck!

I like what @Coloma said, although I’d talk to her in terms of it jeopardizing her job. And I would tell her flat out he is not to attend company events. I understand she is a great employee, but this actually puts her judgment in question.

I don’t understand why you need to see her outside of work?
The other issue I would discuss in her review is disclosing company business to her spouse. Does everyone bring spouses to these functions or just her?

@Judi – we have work happy hours or dinners or just generally get together for team-building events. Not everyone brings partners but some do. I see her to give her mentoring and career advice and that’s where he shows up too. I am also concerned that he exhibits such controlling behavior, but that’s out of my hands and really is for her to figure out.

Could he be jealous?

This guy’s behavior would really piss me off. I agree with @Coloma, just be a broken record and walk away. I am curious how the girlfriend or wife or whatever she is can tolerate this crap. She must be very passive and he must be very aggressive.

Tell her that if he continues his behavior that as much as you like her, you will not be able to socialize with her outside the workplace.

I feel sorry for her. Imagine being his significant other!

He sounds like a boor and a bore. What a jerk. You have the right to tell him he’s not welcome at such events anymore. If he comes, he’ll be escorted off the property (but you might first want to talk to your attorney about this before you do anything). And as much as you adore your employee, his behavior is jeopardizing her job. She needs to get that concept, at the deepest possible level. It’s sad.

I like the first two responses best, from @Coloma and @zenvelo.

Perhaps you could steer him toward another Fluther post.

I would ask her if she likes her current employment and . . .
TELL her to stop talking about confidential information with her partner. Tell her he cannot keep an secret and will cause her to go looking for another position.

We’ve been working together for almost 5 years and am really proud of her development and growth – and feel our professional relationship is worth being honest with her one last time. Thank you to everyone for your advice!!

I’m going to have one last talk with her, reiterate the boundaries and let her know that I really question her judgment in this area. Also, he is not to attend any events until her can learn discretion and tact. I also need to talk to HR – I’m not sure if we’ve ever had to ask an employee not to bring their partner to a work event.

@figbash It is not that the employee can’t bring their partner, it’s that her partner is not appropriate at company functions. They should don’t spill the company info, even if it is her salary in public. It is not in the company’s interest if she has confidential or company secret information available to her to have a security breach because of a significant other’s loudmouth.