Actually after I wrote that first comment which I admit was meant to put her off I thought about who a heckler was. People who feel "part of a group" never heckle. It's those who feel they are "outside the group" that do. You might want to politely include her in the singing. I realize I don't know this woman's personality and this suggestion may be a horrid one to you but, if asked nicely, you might be able to turn the situation around. Don't make it worse by saying, "Well, if you don't like it, let's hear you do better." Instead you might say something like, "I'm sure you've heard some great songs in your time. Would you like to share one with us?" If she hestitates, go on and suggest a song asking her if she knows it. Maybe you could get the group to sing it with her. If this works, I'll bet her heckling days are over with this particular crowd.