Give the female a place to dig out a cave. I use coconut shells and small flowerpots cut in half lengthwise, also driftwood partly buried with hollows on the underside. The digging seems to be a stimulating act for her. Use a fine substrate, sand or very fine gravel. These are bottom sifters with relatively small mouths, so feed small pellets or live blackworms (which should be fed VERY sparingly). They do like chasing Daphnia, though.
Sometimes they just don't get along. I've had females kill males that were too small, they should be bigger than her. I know one person who had to swap out mates 4 times before finding a compatible pair.
If you can separate them for a while, you might be able to grow him out. Males grow faster than females, so he will catch up and pass her in size given time. Easy thing is usually to just get a larger male.
Well I had both of them since they were juvies and were told they were both females so yea... I'm I'm the process of cycling another tank and putting some wild-caught Pelvicachromis Taeniatus in once it cycles. Maybe I can put the male in there with them.