I used to work in this arena. My advice is to tell your child you believe them and are really glad they told you, you will be ready to help them if they want to take it further but their body is their business and you will respect their privacy and their choices.
If they would like, you can put the stuff they were wearing in a plastic bag and put it away (if it hasn’t been washed) so they don’t have to look at it and if they do want to report at a later time it has been preserved.
It’s a good idea if they’re up to it for them to write down in their own words what happened and how they feel. They can put it in an envelope and put it away for the future if needed, but can help to get the feelings out, and to see it written down can help to understand what it was and to process it.
You should write it down too, and put it somewhere safe. On top of this, journaling would be good for both of you. Please do contact Rape Crisis/Stonewall etc for help.
Your child will go through a grief cycle (anger/denial/bargaining etc) and may ‘act out’ sexually or otherwise. Being attacked by a friend (and someone from a community you trust) is like insult to injury as the betrayal and injustice and hurt can be overwhelming and survivors are more likely to blame themselves because it wasn’t a chance encounter in an alley kind of thing.
If you do report it, they should have a specialist sexual offences trained officer assigned promptly who will guide them through choices and help them make a statement or record of what happened.
They can talk to someone at a sexual assault referral centre (SARC) or the GP for some psychological support. Unfortunately, sexual assault is very, very common, they have done absolutely nothing wrong and any guilt or shame is not theirs to carry.