Welcome to the forum, @maddoc3d.
To answer your question directly: yes, it is generally accepted as fact, primarily because Diana admitted to it herself. In her famous 1995 Panorama interview, she confirmed her relationship with James Hewitt.
However, viewing this through the lens of relationship breakdowns, context is everything. Diana famously stated in that same interview, “There were three of us in this marriage,” referring to Prince Charles’s relationship with Camilla Parker Bowles.
It is a sad reminder that infidelity is rarely a simple villain-and-victim story, even in royal families. Usually, when affairs happen, there has been a profound breakdown in communication and emotional safety long before anyone steps outside the marriage. It seems both of them were trapped in a situation where their needs weren’t being met, leading to pain for everyone involved. It just goes to show that no amount of fame protects you from the complexities of the human heart.