My (M) GF (F) has an urgent timeline for kids and wants to start a medical fertility process with a serial cheater. How do I handle this?

Hey everyone, I’m in a really tough spot and could use some perspective. I’ve been dating an amazing girl for about a month now. Due to her age, she has a massive, urgent desire to have children. Because of the timeline she has in mind, it's already putting a huge strain on our very new relationship.

I’ve told her that I absolutely want kids in the future, but she wants—and feels she needs—to start right now. Right before she met me, she made an arrangement with a guy to get pregnant by him. The catch? This guy has a long-term girlfriend who has absolutely no idea. On top of that, he is sleeping with at least five other women behind his girlfriend's back.

When my girlfriend and I started dating, she paused contact with him. However, because she has difficulties conceiving, they still have a hospital appointment scheduled for this coming September to officially start the fertility/donor process together.
I am having a massive moral and emotional dilemma with this. I think this guy is a terrible human being because he abuses everyone's trust, especially his girlfriend's. What hurts me the most is that my girlfriend keeps defending him and his actions.
I’m genuinely trying to be constructive and have already proposed several compromises that I *could* live with:

1. Seeing how our relationship develops and eventually starting the process with me.
2. Going through an official clinic with a completely anonymous donor.
3. Using her gay best friend as a donor, since he has already offered to do so.

I can find peace with any of those options, except for that specific guy. She knows where I stand, and I know where she stands. We’ve decided to take a one-week break from communicating to let everything sink in before we talk again.

My dilemma is twofold:

\- If she ultimately chooses to go ahead with *him* in September, our relationship is over. But if that happens, I feel a strong moral obligation to inform his girlfriend about his serial cheating and the fact that he's trying to father children outside their relationship. His girlfriend is actively being exposed to health risks (STDs). Would I be the asshole if I told her?
\- Is it even realistic to hope we can move past this after our week of reflection, or is this already an insurmountable red flag just one month in?

Thanks in advance for your advice and insights.