If he didn’t show you then say nothing and keep an eye on it. A conversation obviously came up about a place/trip or waking up to the sunset. If he did show you then ask where it it was taken at and what made her send this pic as casual discussion. However, this would be the time to consider and planning to pivot the business. If your business depends on flirting to maintain a relationship at the expense of your marriage what does that say about your relationship and your business? Research other resources and make sure you have access to those messages, all devices, and phone bills.