Do not feel any guilt! First they know the bill is marked and if they have a problem with it change it in Canada before they leave. Unless you were standing at the airport security entrance handing out money I am sure the person would have had enough time to exchange the bill in Canada. People just like to complain or cry the blues ...what we do here is legal and tuff noodles if they do not like it... THIS is only my own personal opinion and should not in any way be taken as the rule of the day.