It is ridiculous.
It was poor judgement on her part to post it, even on her personal account, but at the same time, the employer should not refuse to hire her, just because of that.
I think, if she was qualified and appeared to get along with the bosses, they should have hired her, then perhaps afterwards remind her that even her personal twitter posts can reflect on the company and please keep that in mind and then recommend gently that she remove potentially embarrassing content.
I side with the employer. Its a warning sign. Don’t need potential sex problems at the office. Its just not worth the risk.