Denying entry is for honest mistakes. If someone is actively committing fraud you need to up the consequences in order to effectively deter others from trying their luck.
If she was suspected of fraud then inform, arrest her and make plans for this suspicion to be tested by trial, no? As I understand it she was "detained" rather than arrested, told outright that she had not committed a criminal offence, and was offered no further insight into her situation or when/where/how she would be released.
