Every case is individual, so there is no universal recipe for dealing with it.Basically, you should recognize your own limits in the situation and the most important thing is to base your actions on the wishes of the person concerned. Listen to the person and take them seriously, clarify their wishes and needs, offer support and point out the BFV's support structures, such as the contact point and contact persons.
It is also helpful to write a memory log and report the incident to the BFV via the referees or independently, anonymously if necessary. You also have the opportunity to testify as a witness in (sports court) proceedings.
In all cases, avoid focusing on the people crossing the line and avoid victim-blaming, trivialization, concealment or protection of perpetrators by trying to find explanations for the misconduct.
If there is a stewarding service at the match you are attending, you can also speak to them. Finally, there is always the option of reporting incidents directly to the police.