You are slowing down her internet connection when you are using the internet but likewise, she is also slowing your connection down when she is using the internet, you are after all using a shared connection.

The wired connection is (allegedly) generally faster and more reliable than the wireless connection. We use both at work but to be honest I can not really tell the difference. If you want to test it, try to download the same (big) file with your friend at the same time. See how long it takes to download it using (your) wired connection and with (her) wireless connection. See if the difference is that significant enough for her to make a fuss about it.