So much BS on this story.
Have a look at this from a live M&S advert for a customer assistant in clothing and homeware. One of the tasks is to: "Proactively engage with customers to understand their needs, make recommendations and deliver remarkable service throughout their visit to store." (emphasis is mine). So this employee was adhering to what M&S expects their customer service staff to do.
There is no suggestion that the employee offered to, or wanted to, or was even qualified to do a bra fitting as has been claimed.
The mother claims that she was stood close to her daughter. So the employee approached an adult and her child to ask if they needed help. In line with the expectations of M&S for their employees. And when they said no thank you, she left them to it.
Oh and the claims that the employee only worked in a different Department is also wide of the mark. M&S have confirmed that “We have written to this customer and explained that our colleagues typically work across all departments in our stores and customers can always ask to speak to the colleague they feel most comfortable with.”
Let's park the fact that it's not been confirmed that the employee was trans. That is besides the point. The point is that a trans employee approaching an adult and her teenage daughter to ask if they needed help (which can be anything from advice on products, directions, facilitating booking a fitting), in line with their employer's job description, and accepting it when they said no, is behaving entirely appropriately.