Having private phone calls/texting/skype is actually more problematic than meeting for coffee in a public place. People will say things on the phone that they would never say in public.My family members who have married have done so the proper way. Phone calls/texting/skype for 3 weeks-1 month. Then the young man and his family coming to the house for a formal introduction. The young man coming to the house until the couple decide to move forward with the nikaah. This can occur for months up to a year.
There's no proof she sinned. You should give her the benefit of the doubt.This girl should have kept her sinning to herself. Allah will forgive you for that sin you keep private and make sincere repentance for.