My mum bounces on the step with her toes on, heels off, stretches it out
Don't bounce!
They're called Alfredson's heel stretches - great for Achilles tightness, and as the Achilles inserts around the heel, probably good for PF.
You need to do the opposite of bounce: you need to hang your heels off the edge of a step or stair, and rise slowly up to tip toes, then drop slowly, to the end of your range of movement - so your heel hangs below the level of the step/stair. Do this over about 4 counts each way.
Make sure your toes don't grip - spread them out flat on the stair.
Also calf stretches.
And lots of work on foot mobility and flexibility. Google "doming" - it's what dancers do to help with strength on pointe. Also look at doing Mexican waves with your toes, or play around with keeping all your toes flat on the floor and just lifting your big toe.
Just lots of stuff to keep foot, ankle, toes, and calves mobile and stretched.
I found walking barefoot as much as possible helps, but a lot of people are told they shouldn't do it.