Although he may have reasons for not remembering, I don't see anything wrong with asking this question especially if you think this relationship is going to another level. I asked my DH all kinds of stuff about his sexual history when we were dating. I also had him get an HIV test once we became exclusive.
Were you guys exclusive or just in the 'getting to know you, see where this goes' phase?
I think the issue isn't the fact that she asked about his sexual history, it's what PART of the sexual history she asked about so specifically, and when. According to her timeline, they've been flying back and forth and having sleepovers and he attempted raw dog twice while they were together.
I don't think you let ALL of that happen, and THEN decide to ask about someone's sexual history.
Wanna know if you're the only one they're dating? Yes. Wanna know where its going? Yes. Wanna know their STD/HIV status? Of course.
But you want to know specifically speaking, when was the last time they got some?
In my mind, that says to me that you're asking me that to either:
a. Make sure you're the only one I'm doing
b. You're a bit territorial and assumed that you'd be the only one I'm doing, (and will get highly upset if you find out that I am in fact not just doing you.)
c. A and B
I think OP has probably caught feelings for the dude and wants to be assured that she's the only one he's doing these things with. Cool, but don't mask it behind a question that CAN be seen as being nosey.
Also, don't expect too much. It seems like they've had such a massive exchange of girlfriend-y/boyfriend-y things going on that the assumption of a presumed 'relationship' could be going on too.