1) No. She does not have to tell, except if the virginity is taken as a condition in the marriage contract or she gave the image that she is virgin.
However, it is better to be frank with him, because he will soon find out that she is not virgin.
2) What type of assault it was? Was it full intercourse, or with finger? If it was full intercourse, or insertion of the penis tip, then she had janabah, and needs to offer the prayers took place since she completed 9 lunar years, till she had the first ghusl. If the assault was with finger only, then no need for ghusl. There is one more thing, she can can get examined by a female doctor to verify whether she is virgin or not, before revealing it to her fiance.