You didn't say exactly how old your female is, but from this statement...
"I seperate and join them every year"
... I am guessing she is at least a couple of years old. Assist feeding at this age is really not wise. The stress already on your animal will only be compounded. Assist feeding is used by some in getting stubborn hatchlings to eat, but once established it should never be necessary.
There are usually very good reasons why ball pythons go off feed. Improper husbandry, stress, parasites, and while being cooled for breeding are some of the most obvious. One thing that jumped out at me from your post is that 75-80 degrees on the cool side of the enclosure may be a little too cool for this female. Upping this temp to 80-82 may get her going again. Another thing may be the presence of the other snake; this is a factor that may be causing stress.
I have also seen balls stop eating because of stresses caused by the following; lack of hide boxes, lights on 24 hours a day, enclosure kept in a high traffic area, and over handling to name a few.
I would not be so quick to rule out parasites either. Two animals can be housed together and both have different levels of parasites in their systems causing completely different behaviors. Your male may only be infected with a small gut load of parasites that aren’t a problem today but will eventually reproduce and cause him to go off feed as well. Parasites seem to be pretty common (even if at minute levels) in a good percentage of captive collections. There is an abover average likelihood that these animals are not 100% clean even if that is not the cause of this female going off feed. Assist feeding is very stressful and traumatic to a ball python at that age, having a fecal done to rule out parasites before attempting something so drastic is in the best interest of your animal.
Even if you have been doing the same routine with her for many years and have not changed a thing, she may have changed. She is a living creature and sometimes animals need something different. Try switching things up. I have personally seen tricks like warmer temps, moving the enclosure, changing bedding all stimulate a sudden non-feeder into taking food again with gusto!
Before jamming food into her mouth, please exhaust all other options. It’s really in her best interest.
Hope this helped. Sorry it's so long.
-Adam