Yes. I just decided to give it a try after reading your message. I connected my Note 8 to my GS8+ using the supplied USB-C cable. The USB-C plugged into the Note 8 and the other end I used the included USB-A to USB-C adaptor and plugged it into my GS8+ and the Note 8 started to charge. There are other options to choose for the USB port and you can choose the phone that should be charged.
That being said, I notice you said S5 and since the S5 has a micro USB port, I decided to try it between my Note 8 and GS7e, which has a micro USB port. I used a micro USB cable and inserted it into my GS7e and the other end I used the USB-A to USB-C adaptor and plugged it into my Note 8. The Note 8 started to charge the GS7e. That is, the GS7e was receiving the charge. There is no way for the Note 8 to receive a charge from the GS7e.
So to answer your question - Your Note 8 can charge your girlfriend's S5 but not vice versa.