Niji already knows it's a woman Remu's getting involved with as per chapter 3, so having a man as her "rival" in her story seems to be a deliberate change. Now I just want to know why she's doing that.
I checked the Raws to be sure, and the way that Niji addresses Azuki in ch 3 is as 「例の人」which would translate into something like, "The person you mentioned before" (lit. person of example).
In ch 2 Remu calls her 「気になる人」(person I'm interested in) and Niji calls her 「その人」(that person). No gendered terms are ever used.
Depending on who you ask, I think one could argue that all of the gendered terms being used in the English translation here is a straight up mistranslation. There seems to be no evidence that any of Remu's friends have any indication of Azuki's gender, and if you assume that the society portrayed in the story's heteronormativity somewhat reflects our own society's, then the more likely conclusion is that Remu's friends think that Azuki is a man with the way the conversation flowed. But at the very least, all of their conversations surrounding Auzki seem to have been gender neutral, with no explicit expectations either way.