The chasing of the female is par for the course as far as platies goes, he will hound her day and night, as african said to the point of killing her with stress.
It is a good idea to keep a ratio of 3F:1M to give the girls a rest from time to time.
If she is looking big and hanging out down in the plants, there is a good chance she is ready to drop very soon.
Your easiest shot at saving some fry is to add lots of java moss or something similar for the fry to hide in, you should get some survivors unless its a full on community tank.
If you are serious about saving most of the fry, you will need to either isolate her in a breeding trap or move her to a separate tank once she gets big.
Keep an eye on her and when she drops remove the fry about 12 hrs after you see the first fry (it can take that long for her to pop them out) and keep them separate in a floating icecream container for a day or so until mum recovers then swap mum and fry over.
if you miss a birth dont panic she will drop again in approx 4 weeks.
One mating will last for up to 3 spawns so If you really want to you can keep her on her own untill you catch a birth or 2
the big thing is to avoid stressing her just before she drops, otherwise she will abort and you get nothing, don't take her out of the water if moving her, herd her into a container or the birthing trap and take her out of the tank water and all.
Phew
Good luck man, join the WAS and I'll be able to help you out in person :bounce: