If she was truely raped by this person, you should not be posting your concerns over weather or not it is his word against hers, you should be supporting your daughter at the local police station, hospital, rape center, while she is reporting this dirtbag!
There is never any justifiable reason for anyone to refuse to accept the word no! This person will continue to pose a threat to your daughter and every other daughter, sister, mother who comes in contact with his sick, unexcusable actions.
How many more girls have to be assaulted before you believe the threat he poses to the public should be dealt with? The answer better be none!
How many other women has this man assaulted? How many other women called this piece of crap a friend? Who knows and who cares, report him now and stand by/support your daughter throughout the entire ordeal without questioning your decision to do so.
He needs to be stopped, yesterday, not today!
If your daughter is not telling the truth, she needs to seek treatment at a mental health facility. There is never any excuse for fabricating a story of such a nature.
Assuming she wouldn't, contact the authorities now!