Sexual harassment
Such comments might amount to sexual harassment, prohibited by section 26 of the Equality Act 2010 (EA), and potentially could form the subject of a grievance and/or claim to an employment tribunal.
In order for your member to be able to raise a grievance or lodge a tribunal claim against the employer, she will need to show that the colleagues’ leaving drinks were sufficiently connected to work/the workplace.
If so, the employer can be held responsible for these breaches of the EA. This is known as making the employer “vicariously liable”.
In the case of Chief Constable of the Lincolnshire Police v Stubbs [1999] IRLR 81, the employer was held vicariously liable for the sexual harassment of an employee which occurred at similar social events.