Dated scope documents, change requests and their approvals, project plans by phase, and a simple log of where the person worked and who directed the work. Contemporaneous records are far easier to produce than reconstructions, and a project with several revisions will not be reconstructible from memory.
Capture the change, not just the current state
Keep superseded versions of the scope with their dates. The history explains why the work looks different now and shows the changes were tracked rather than ignored. Where a change was reviewed and cleared, note that too.
This costs a folder and a naming convention, and it is the difference between explaining a project's evolution and simply asserting it. There is a cheap habit that produces most of this evidence automatically. Ask the project to copy the named owner on every approved change request, and file those emails unread in a single folder.
The folder becomes a dated record without anyone maintaining it, and the only manual work is a short quarterly note recording where the person worked and who directed the work. Reconstruction, by contrast, is expensive and unconvincing: a chronology assembled after a question is asked tends to describe what people believe happened rather than what the documents show. Store the folder somewhere that outlives the project, since the person most likely to need it later is someone who was not there at the time.