who knows man everyone's different we all have different personal boundaries for different reasons
maybe she's just not ready for that yet(idk how old yall are but im guessing teenager somewhere), maybe it's tied to personal trauma, maybe she doesn't feel comfortable enough around your friend yet, maybe shes asexual, maybe god intervened and spoke to her and thats why she wasn't okay with it
no way to say for sure this isnt like something you can put on a flowchart, id just encourage you or your friend or whatever to keep an open mind
probably dont pry either(goes for both of you), if she wants to explain her boundary or if it changes thats kind of up to her to decide that in time