My Long-Term Partner's New Flirtation - Advice Needed?

So here’s the deal, I’ve been in a non-monogamous relationship with Sarah for about five years now. We’re both in our 50s, and while we started as open to flings and casual dates, over time Sarah has become more selective and tends to only pursue deeper connections. I’m still down for pretty much anything, but lately she’s been hinting at some concerns. She thinks maybe one of her new flirtations is moving too fast and that it’s not healthy or safe. Here’s where things get tricky, this other guy is significantly younger than us both, around thirty, and he seems to really like Sarah a lot. It’s not that I’m jealous; actually, the idea of them getting closer makes me feel more secure in our relationship with each other. But Sarah worries she might be taking advantage by getting involved before things are officially serious, especially since he’s so much younger than her and me. He has been persistent and they have exchanged a lot of explicit messages. I think it could just be innocent flirting or maybe even a bit more than that between them, but nothing too serious yet. The problem is Sarah hasn’t given him a clear indication where she stands on exclusivity. It’s gotten to the point where he wants to meet in person and spends hours messaging her daily. I’m not saying it’s bad, just wondering if there are any red flags here that we’re missing? Or should we just encourage them to get together properly since they seem compatible? Any thoughts or advice would be greatly appreciated, as this feels like one of those “Am I the asshole” scenarios where everyone might have different views.