Depends. Is this a friend they can hang out with anytime or someone that isn't normally in the area? We are completely trusting in our relationship to let the other person have friends of the opposite gender. In fact we've both continued using the dating site we met on to email the opposite gender looking for just friends. I also answer craigslist ads in the platonic but m4w section and my husband knows. It's not an issue because we know the other person won't do anything. So the only thing in your situation that would matter is that plans are being cancelled for someone else which is not right if they can hang out with the person anytime but totally understanding if it's someone they rarely get to see.