My suggestion.....do not deal with your bf's ex at all....
Let your bf deal with her by declining the offer and telling her straight that he's not interested to take her out anytime or ever.....
atleast he owe you that, being the woman in his life now....
just my opinion though..
Thanks Sophie, I am looking forward to his reply to that girl. He can't go on to FB cos he is at work. He doesn't have any idea what this girl posted until I told him. I just can't believe some girls would actually do that.