First of all, you should check whether there is any conflict between your network and IP. I believe you have used a switch or router. If yes, check whether there is ARP attack in the log. If she uses you, she will be dropped, IP will be dropped, and MAC address will be dropped. Another way is that your or her system is poisoned. The easiest way is to use an external USB network card and surf the Internet with her machine. If not, it is judged that there is a network card to be processed.