Frenchman's Flat is currently a parking lot, and picnic area (No Camping Here for 20+ years). It is Not open 24 hour. Dawn to Sunset. It is on Piru Creek and it is possible to walk upstream to Pyramid Dam (on a closed paved road) or downstream on fisherman's trails. This area has special fishing regulations that you need to understand BEFORE the game warden cites you. You need a Forest Service Fee Area permit (Guess what? You pay the Fee!). Fun place to take my dog to splash around early mornings. But in afternoons it gets crowded with people who don't read signs in any language, drink beer in public (and mark territory outdoors) and leave illegal fires unattended.
Overall, it is a popular area to enjoy the forest.
There is camping available nearby - about 2 miles before the Frenchman's Flat dead-end, at Oak Flat Campground. It is Not on Piru Creek and does not have any stream.