I mean how far aware from her and her soul energy is he? We know that he needs to feed off of it or fire in order to power himself. Hence the questions regarding her catching up and his power levels/Nami’s ability to counter him. I do realize that his personality is different. But the question was one of physical proximity as we learned things about the limitations from the previous arc. He could burn the surroundings to create fuel for himself, in theory. As, once the fire starts, it will no longer be a part of him, this available for consumption. However, if there’s a body of water, that can be channeled effectively towards him as Jimbei was able to do, then, he can be dampened. So, more powerful rain attacks, the whirlpools, etc could be utilized. I’m guessing that they’ll end up near the bridge or one of them, to give Nami an advantage. I don’t see why she wouldn’t use similar attacks to the ones I’ve made up in the User Tournament. She has Rain Tempo, the various Zeus attacks and she has the failed Snow Storm that only failed because Giolla transformed it but it proves that she’s incorporated the wind technology so she can create hurricanes/use the wind in combination with the water if she wants to in order to attack Prometheus. But all of this depends on him not being constantly fed Big Mom’s powerful soul energy fresh from the source/her not being right there for backup. These are the points that I was trying to make.
Even if he is close, there is still an "energy consumption" aspect. There is the scene of him literally burning out and reverting to the chibi version of himself after the large attack (probably the Homie-based woods). But the same thing applies to any logia user anyway. The problem is that even with the "weakness", the power gap is too big. My concern is that Nami has only displayed thunder based attacks. She no longer uses the wind/storm aspects in her combat (in theory it should still be there with different weather eggs, but it hasn't been shown yet and I would think Oda's saving it for a climax fight). The rain to soften the crackers wasn't a downpour. Even that probably wouldn't be enough since Prometheus just evaporates the cloud with the power level difference. So yes, if Jinbei was there, that's a different story since we have someone of the same tier. The water volume has to be massive. A huge fireball is still going to stay lit in rain. Or maybe a better example is trying to stop a flamethrower. There has to be sufficient coverage to choke out the flame completely.
She herself admits that Zeus produces much stronger attacks than what she can do. The best thing Nami can do is negate Prometheus with Zeus. With the threat of Big Mom getting angry, would Zeus stay loyal for only food? Since Big Mom rules through fear/oppression, Zeus' personality implies he's going to realize he's in for bigger punishment once Big Mom is present. In theory, Zeus gains the upper hand with Nami being the source of additional energy. The main question is if Prometheus loses all heat and can be safely touched once he exhausts all of his energy. If so, some sort of capture makes sense. He won't be a willing "slave" without a change of personality. I would think something along the lines of Prometheus going to Big Mom and reporting Zeus' location upon him losing the upper hand in the fight. He's the rationale one, so he's going to prioritize the goal of getting Big Mom back to full strength (he knows Big Mom is currently at a huge disadvantage if she has to go against Kaidou if things fall apart => Big Mom being at least one of Kaidou's 19+ defeats is likely).
It would be huge fan service for Nami, but all the breadcrumbs indicate something else is going to happen. We are in for an interesting fight and the expansion on the energy consumption aspect is something I'm most interested in. I guess my main point is that we have a whole lot of character development on Prometheus despite limited panel time. This forces scenes with him to go down a limited path of options. He isn't going to go for an all out attack since he knows he underestimated the Mugiwaras the first time. If their personalities were switched, then Zeus might be stupid enough to fall for the same trick.