Again the authors are planning things in advance.
We learn that tracking magic through spells is the domain of the Goddess, thus explaining why Frieren hasn't employed it yet, before we discover that Sein's also walking in that City. Then, we see Frieren use that same knowledge to give Sein a sign that they're here and might need help, which Sein immediately picks up on, and thus they get the best helper to deal with the current situation.
All in all, this is showcasing that the world they're in is actually a "living" world, where people move around on their own and actually have agency. Is it convenient Sein is there at the same time they are? Sure, kinda? But it also tracks with the fact that he's on a searching quest and that Frieren, Fern and Stark spent a bunch of time doing the mage's exams, giving him time to reach the same city.