I originally used myself as the 1st as i was the one enquiring, then x was the 7th as he is the one i'm asking about, as between us its gone a little more further than fun and sex, and this other person i've found out about, she gets the 5th. I mean he's married too, so this makes it even more complicated on who's the 7th.
If you are asking about his relationship with K, you have two choices
My preference would be the same as CosmicBeing. As you are not directly involved in the question, Give him the first. Now you have a decision about K. Does she get the seventh or the fifth? You know he's married, she may not. Do you know for a fact that they have some sort of relationship already going?
If you really don't know much about what is going on, she gets the seventh, as simply another person. If you know he's having sex with her then there's an argument that she takes the fifth as a casual fling. There's also the question of how he looks on her. If he sees her as a potential source of free sex then she's fifth house, at least at this stage. Do you know what he's told her? Does she expect more than sex?
If the question boils down to does he want to continue with her, then she's seventh. But note, you are not asking will he marry her or will he move in with her. He can't do either and you know that. But you can use the seventh for a question such as has he any feelings for her.
Her significator in the first simply means she's offering herself to him not that he will choose to accept her. But he can choose to accept if he wants. The condition of his significator will answer that. I've just looked at the chart and Venus is only a degree from the second cusp and in a different sign to the Ascendant. Medieval Astrologers would treat Venus as second house, not first, when using quadrant houses. So she's not offering herself, if he takes first. She has little interest in him, though I haven't checked the minor dignities.
However if she's interested solely in sex that doesn't of itself rule out a second meeting. That there's no aspect forming between Mars and Venus may well rule it out.
Your second option is to take the first for yourself. You're asking about him as another person, rather than as your lover, so he gets the seventh. K is a possible rival for his favours, so she gets his fifth house, which is the fifth from the seventh of the chart you cast - that is the eleventh house.
In the second approach you are testing to see if there's a casual relationship between them and whether it will last.
Both approaches are possible because they answer different, though related, questions. So your first task is to decide what question you want answering.