...I do not know how to respond to inappropriate behaviour that I am involved in?

It may be very difficult to respond, and every person may feel a different level of stress in such a situation. However, if you are aware that the behaviour you noticed was not all right and you want to help to prevent it in the future, you can choose one of the following possibilities: 

Direct confrontation  

Its objective is to end the situation; it is good to object assertively to the unpleasant behaviour of the originator. If it is a grave situation, we can involve more people (for example people who are around, the receptionist, lecturer…). The best thing to do in the direct confrontation is to object as the first person and say that you do not like such behaviour. For example, "I think that the way you are treating Helena is not appropriate, stop it. “ 

Indirect confrontation  

In indirect confrontation we can take the victim out of an unpleasant situation (e.g., to make up that you need their help immediately). An effective way may also open conversation with the person who is being harassed and ignoring the person who is behaving inappropriately (e.g., a question about time, asking to pass something). 

Postponed intervention  

We can show our support to the victim rectroactively, say that we are sorry that we were not able to respond to the situation at that moment and offer support if the victim wanted to solve the situation. Similarly, it is possible to confront the originator retroactively and say that such behaviour was not appropriate.