Why hasn't she spoken up? There is no reason other than a bit of embarrassment for her to lie.
To be fair having to tell your Boss you drunkenly shagged a man you just met in a public place (which is also an offence) would be more than a bit embarrassing but I agree the vast majority of people would just come clean in that situation before it got out of hand. It's consistent with her character that she doesn't though as we've seen her doing things like turning up to work late and hungover, driving when clearly still over the limit, using her phone while driving so we know she's a person whose judgement is seriously flawed. Her mum also makes that comment about how she needs to remember that her daughter is her responsibility and both her kids are clearly very unhappy, suggesting she's struggling to hold it all together at home too. I got the impression that her life is quite chaotic and things were unravelling personally and professionally. So it's conceivable that someone like that could make really poor decisions under pressure.