Well of course it is possible that it might be his, who knows if she is even being truthful about her cycle dates. If it was a week after her period started that they last had sex then it probably isnt his. Ovulation is usually later in the cycle. Not all women are the same though, and it could be that it is his. The only way to know for sure would be a paternity test. There might be a way through the courts that he could order a paternity test be done.