Male boss got changed in front of my female colleague, is this legal?

So in the office, there are 2 the co-founders (men, 30 and 50) and 3 other employees (3 young women in their early 20's). Whilst two of us were in the next room, my other colleague came in to say that she caught our boss (the guy in his 50's) getting changed whilst staring at her. She didn't see anything as he was behind some rails, but she caught his eye and waved when she saw him staring at her, because she didn't know what else to do. He waved back and continued to stare whilst still changing his clothes. This made her extremely uncomfortable and we all feel quite weird about this. He does usually get changed in the bathroom so I don't know why he did it in front of her.He's generally a super creepy guy anyway, he usually stares at you for way too long anyway when just talking. He's also previously said that he's a "hugger" before the work Christmas do. Because the company is such a shambles, we don't have a HR department to report this to and we can't tell the other boss, as he is less senior. There is a lack of professionalism in the envrionment anyway, however this steps over the line for us. Does this count as gross misconduct? We are all Kickstart employees, so should we go to Kickstart and tell them?