Sadly I don't think this is that uncommon. My husbands ex wife is like this. One of the recent times my father in law was in town he wanted to take my husband, me and the kids out to dinner. She invited herself and her boyfriend of like one month out with us and my in-laws. She used to invited herself and her second ex husband everywhere too (and never even once offered to pay for themselves! She always assumes if my FIL is in town she gets free dinner, even brags about it in FB!) Even though she cheated on my husband with her second husband (who she later cheated on with her boyfriend).
She even managed to tag along with when my FIL took the kids to Disney. She invited herself and her second husband (before they divorced) and had my FIL foot the bill. She "bought" me a tshirt on that trip. I recently wore it to one of the kids bday parties (which we paid $600 for and she contributed nothing to but showed up and took credit for anyway). Since she was there I thanked her again for the shirt. She asked me why and then basically admitted my FIL paid for it and all the "gifts" she brought back from the trip and she didn't even remember it or what she got for people, she just gave a pile of souvenirs to my FIL and told him to pay for them and then give them to everyone and say they were from her.
Last year we had to have Thanksgiving a day early because she told my FIL and MIL we were all eating at her house when we had no plans to eat with her and her boyfriend. She wanted her ex FIL to meet her new boyfriend though, so she told my in laws we were doing Thanksgiving out her house. He was not amused when he found out myself and his son would be there and she had tricked him and his wife into RSVPing. We held thanksgiving the Wednesday before so he could spend it with his son and the kids. He complained about having to go to her house the next day but didn't want to back out since the kids were now expecting him and we're excited to show him their new rooms (their mother had just divorced and moved out of their step dads house a couple weeks earlier)
The crazy thing is my FIL hates her but she thinks he adores her. He tried to convince my husband not to marry her (compared to him giving his blessing for our marriage) and tried to convince my husband to leave her long before he did. The only reason my in-laws deal with her is because until recently she had majority custody and he had to placate her to see his grandkids. For the last couple of years we've had the kids the majority of the time so my FIL has tried to back off contact with her. She doesn't get it though and still thinks he's her FIL. She even calls him just to chat, which he isn't the chat on the phone type of person but he doesn't want her to say upsetting things to the kids about him so he plays asking for her.
We've all agreed the minute our youngest turns 18 we are going NC and blocking her from every form of contact possible.
Some people have no sense of boundaries and are incapable of realizing when they aren't wanted.