I think it is fake.

Like many of these hoaxes, it's just too perfect.

Not only did she supposedly catch him after purchasing both a GPS tracking device and expensive camera, but then used their "investment account" to buy the billboard.

Is it possible?

Sure.

Is it likely?

No. Seems straight out of a fiction writer's dream.

The biggest red flag is the investment account. That sounds like a sick burn to an outsider, but in reality, it doesn't matter which account the money came from. If a wife spends money on a billboard to call out her cheating husband, it comes out of the family money -- regardless of whether it was a checking account, savings account, investment account, or cash in a shoebox under the bed. There would be no reason to state that, as the husband would know right away that the billboard was bought with their community property. This is more something that someone would write if they were trying to make up a scenario in their head to get the public's attention.