Am I right to be skeptical?

Yup. Of course.

But I have also seen some people improve (as individuals, as for their business/productivity that's up for debate) after being with a highly rated life coach.

She doesn't listen to my advice because she claims I'm blinded by frugality and that is an important investment

This is more a relationship issue. If you play an important part of her business, then this is an issue. If you do not, then just back off a bit.

Who could I get her to talk to?

Other people in the same business/industry. Groups on facebook, reddit, old forums.

You don't have to say specifically, but what industry is your wife's business in?