Hi, Bunraku--
There is a whole lot of horary astrology that I still need to learn, and I can't say that I follow Lilly closely, as he has so many testimonies that sometimes it's hard for me to make sense of them all. The testimonies you cite sound more like the BF did have another GF once, but that she's not part of the picture now.
Here is my rationale:
1. I was informed by some good horary astrologers at Skyscript that I should use the 7th, not the 5th, even where the relationship is new or uncommitted; but that if a significator falls in the 5th that may be useful information.
2. She's Saturn, he's the sun. The sun applies to an opposition with Saturn, but before it does that, it hits a trine with Jupiter.
3. The moon (her emotional stake in the matter) has passed its last aspect with the sun. The moon applies to a sextile with Mars (the "boy planet") but is this a strong enough testimony? Mars is in its own face, but otherwise lacking essential dignity. Venus is in detriment, picking up no other essential dignity.
4. If we follow your explanation of Lilly, the moon has passed Venus, so what ever happened in the past is staying there. Venus does not perfect an aspect with the sun.
5. The moon translates light from Venus to Mars, but Venus is in a weakened position, and Mars isn't such hot shakes, either.
What bothers me a bit is that the chart doesn't tell a story that I can make sense of. Why is the BF's significator in the 4th? It's his turned 10th house: maybe he's focused on his career? Why is her significator in the 11th house? It's his turned 5th: maybe he thought of her as a light short-term relationship to begin with? Why is the moon in the second house: money troubles?
Either way, I don't get a strong testimony of another woman in the picture: more like, what actually holds this couple together? And read her OP: she's already thinking of moving on.
I'd enjoy seeing your interpretation, Bunraku-- as well as zodiacangel's reply.