Mmmmmm.....I think Diana ‘messed around with the horse stable man’ as a retaliation against Creepy Charles.....who was long messing around with his lowlife squeeze (who he was caught telling he wished he was a feminine product of hers). Gross.
I think she thought she could somehow make him jealous, and, change his nasty ways.
All people who cheat will always have their reasons for doing so. .....however anyone who hooked up with Charles was going to be in for a rough ride just being married to the creep. He wasn’ exactly a prize.