My mom is leaving me to go see her internet boyfriend?! Please help!!?

At first when I found out about this I didn't think it was going to go so far but this is just getting terrible! This man is married with a wife and kids. Last Christmas, instead of getting me a gift, she got him a $500 giftcard and couldn't buy me anything cuz she had no money left. This is really the last straw. On February 25 my mom is going to buy a 300$ Plane to go meet him in Philadelphia. I really don't know what to do. And she hasn't told me about him yet but it's really obvious. I'm 13 btw :/