What to do? My good friend is in a relationship with a nice guy whom I have dated in the past and she is now inviting me to her place?

You have very friendly nature and you dont want him to create a bad scene if you all three are together, so just tell her what you feel about that guy and be honest. If you still have feelings for him you wont be honest for your friend. So be honest, thats the best policy always.

If you broke up years ago:

Do you still have feelings for him? If so, are they strong enough that you would do something to sabotage their relationship if given the chance (don't answer immediately. REALLY think about it)? If so, I would not go over there or have anything to do with the situation until you've moved on. If not, then go ahead like you would if it were anyother boyfriend of hers. Either way, be honest with your friend if you are not comfortable going over.

If you broke up recently:

That's pretty rude and inconsiderate... Is she one of those girls who would just want to rub it in your face, "Haha, I have him and you don't"? If so, don't go, and don't be friends with someone like that.

With friends like that who needs enemas? Talk about rubbing salt into an open wound.

Wow, she’s practically slapping you in the face lol.