1. Yes, it's still closed during the week, and open on weekends.
2. The parts that aren't open yet aren't marked (nor some parts in Homestead that are), so she'd need some kind of instructions.
Roughly: Go to the end of the Baldwin Borough Trail. Continue 800 feet on the railroad ballast under the Glenwood Bridge to the road (Haysglen Street) and turn left, then right into Sandcastle. Go through Sandcastle. Get on the Waterfront Drive sidewalk and bear left past Costco and the multiplex. At the pedestrian bridge over Waterfront Drive, take the bike trail on the left, and when it ends, continue left on the Waterfront Drive sidewalk, around the big blue building (Marcegaglia), and get on the trail again. Continue on the trail to McKeesport and follow the markings along Locust Street, down the sidewalk on Lysle, and right into the trail parking lot under the Fifth Avenue Bridge, where the GAP resumes. I think the McKeesport section is pretty well marked.
3. 11 AM. http://www.sandcastlewaterpark.com/site/hours.html
She may want to print out the PDF map from here first: http://www.steelvalleytrail.org/trail.htm
Also, Google Maps in bike mode now shows the entire trail from the pedestrian bridge under the Homestead Greys Bridge down through McKeesport.
(Edited to add details.)