I'm a non-native speaker of English, so this might or might not be something very basic. Nonetheless, it's baffling me and I'd love some help.
A friend of mine wrote this sentence in a story for which I'm a pre-reader:
They crossed a highway along the river shore and then a bridge, leading them to a dirt road winding down a thin peninsula that jutted into the river.
In my mind, based on how my native Portuguese phrases things, I felt that "leading" gerund there could be replaced by the determiner plus simple past "what led", and the phrase would "sound" better:
They crossed a highway along the river shore and then a bridge, what led them to a dirt road winding down a thin peninsula that jutted into the river.
The "what" in there is supposedly a determiner referring to and meaning the whole of "They crossed a highway along the river shore and then a bridge".
However he told me, and I quote him, "your suggestion in this case would cause the sentence to make absolutely no grammatical sense whatsoever."
I'd like to understand why that's the case, what the exact rules are, and whether there's a correct way to do a gerund to simple past conversion in this and similar cases.
Thank you very much!