Re: "Is she cheating on me?"
The ascendant is very late which implies something is too late about the question, he already knows the answer or something is about to change.
He is venus and the moon and she is mars.
The moon is very strong in cancer in the 9th showing maybe maybe a philosophic response or maybe even a legal response.
She is mars on the apex of the chart. She is not applying in aspect to anything right now so if there was anything it is over. She was applying to the sun (traditional ruler of males) but it appears it was a friend only as the sun is inher turned 11th house.
There is Saturn retrograde in her turned fifth house..maybe someone from the past,...maybe older who has shown up again fairly recently but there maybe have been only a communcation as Saturn rules third.
These things are really tricky and I am not an expert on this so I am just putting my two cents out there. There are several ways to approach the third person idea and they are still a bit confusion to me .