This is obviously someone very close to you. Can't see that there is anything potentially illegal about sharing an unsecure wireless network as long as you don't access anything on another PC.
There is every possibility the person may not know unless they bother to check who is logged on to their network (shows the worth of allowing trusted pc's only to connect!) or they catch on that their download usage looks high or reach a limit they hadn't expected. The question is - is it morally right?