If you see someone being sexually harassed, intervene. This could be by directly calling out the action or immediately warning security or a member of staff about the situation.
If you are worried about someone who has been affected by sexual violence or harassment, you can fill in a Safeguarding Form and warn a relevant member of staff.
You might also suggest your friend seek help. The University offer free Counselling Services and their services can be contacted via their webpage. Alternatively, you can guide them to services on this page for support.