So basically the case is this. We've been together for almost a year now and although a lot of her defensive walls are coming down, she is still very defensive about a lot of things. 

I recently found out that she's been going on random car rides out of town with a single male friend that she hadn't told me about previously, and is now straight up not allowing me to meet (**major fucking alarm bells going off here for me**) but she is 100% adamant there's nothing going on between them. They apparently mostly talk about shared interests (like nature, climate, green tech etc) but she once mentioned he made comments about his recent sexual conquests. 

In our previous discussion about this she has alluded to the fact that if she wanted something to happen with him it could, but she doesn't. I asked her if that means that he either has done or continues to express any kind of interest and/or willingness to sleep with her, and she just rolled her eyes and absolitely refused to answer the question. 

While I'd like to believe she truly hasn't cheated on me yet, but I am just unsure. 

Now given what I've read here about BPD,  compulsive Sexuality, and self destructive behaviours, am I right to be worried about something happening here, either now or in the future?

Would it be out of line for me to give her an ultimatum along the lines of "these secret drives stop, or we're done?" 

I've also been wondering if her recent behaviour of occasionally withdrawing from me, a lot of the time, questions why I'm with her, suggesting we open the relationship (I'm 100% monogamous) etc etc etc. Is a sign she might be looking for an out to be with this dude guilt free.

I'm writing this last thing at night while tired so I apologise if this wall of text is littered with errors. Any help you can provide would be amazing, thanks.