It's texting, so even if it wasn't your characters, but you who did it, you wouldn't be having sex, you'd be text chatting about sex, which is very different.
I wouldn't have minded if my girlfriend did it, especially not with some dude she hadn't even met, but just talked to online.
if you want to do it, but is afraid it'll cause her problems, ask her aout what her boyfriend would feel about it, who knows, he might be okay with it.
After all, it's not like he doesn't watch strange naked women online anyways (i know, i'm assuming, but i'd be supprised if he didn't watch porn)
Ofc, if you plan to meet her in person later on, her boyfriend might not be thrilled about.