You know the saying - give an inch and they'll take a mile -
most people won't take advantage of your good nature but every now and then someone is going to try.
In some cases it works to their favor and so why not try it again with you or someone else. Can happen anywhere. I say give the benefit of the doubt, if your asked the first time and if you refuse but are still asked again - then you make yourself clear and if need be - end the "friendship".