It will come down to her word against his - unless there is any other evidence, such as evidence of recent complaint (eg she says to a friend/parent in tears that she was raped). He should not answer any questions under caution unless he is represented by a solicitor and I would advise that he gives a full account rather than going "no comment".
The question is, can the prosecution prove it beyond reasonable doubt? On what you have said, it seems unlikely. Possible but unlikely.