There is also no way to know if she had sex with more than one man around the time of her conception.
Get a DNA test and a lawyer. Unfortunately, in some states and countries, if he claims the child by putting his name on the birth certificate, it won't matter if it's his or not. He'll have to pay child support. He needs to get a lawyer and know his rights. If it is his child, he needs to be a father. If it isn't, he needs to know and the real father needs to step up.