I’m so sorry to hear about her friend, keeping them and you in my thoughts. 
As an educator, one resource I really like to use is Sesame Street in Communities - they have advice and guidance for handling every topic from Incarceration and Deportation, to Autism, to Health Emergencies, etc.
I’ve used them for guidance and activities to help with a few things before, and since the site and show are run by a team of childcare professionals (teachers, pediatricians, psychologists, etc.), I find it to be a trusted resource.
Otherwise, I’ll echo that you need to be honest but in an age-appropriate way, and that she may have questions, OR she may not ask at all and seem to not be phased. Both are completely normal responses to a trauma like this from a child so young. The seven stages of grief may or may not apply here as well, as they may be grieving how things were before.
In young children, sometimes grief can cause a regression in behaviours, such as returning to thumbsucking for comfort, or issues with potty training, irritability, etc. All of this is completely normal, and not permanent, but a way of their minds processing everything and dealing with big emotions and feelings they’ve never dealt with before.
Hope this helps somewhat, take care!