However, the male guppy is rarely a threat to the fry - with the female emitting a pheromone encouraging him to court and mate with her, he has other priorities in his mind (such as it is).

However female guppies can be quite effective in eating fry. The other female may have been even more so than the mother. If you want to save more fry from the other female, remove her now slimmer "sister" to the 55. As I'm sure you have read, feed the expectant mother very well with frozen blood worms or white worms (defrosted in cool water and rinsed through a net).

Or if you have a plastic leftover container, perhaps 6"x6"/ 13x13cm, which you can dedicate to storing a portion of rinsed black worms in the refrigerator (maybe under the cold-cut drawer so others need not see them) then go for the "worm bowl". The worms will need to be frequently rinsed (I use soap-less, well rinsed, gallon pickle jars, the bathtub spigot which is not too violent and the toilet for dumping when no other customers are near the bathroom) and casualties removed every day or three. Just cover the worms with cold water. Don't make it deeper - but don't forget about them either. ;)

You will also need a turkey baster (also dedicated to things fishy) and a 1/2 or 1 pint soap-less pickle/jam/salsa/mustard jar. (I cheat, rinse the jars, soak them in a bleach solution and declorinate them in another container - we are playing a 10-gallon request for another fish head at the moment.) The jar is placed on the bottom of the birthing aquarium and a few worms, beyond what she has eaten, are placed there. (Don't try this in water with much salt because the worms will die and really foul the tank!) It is a rare well-fed female who will prefer her fry to blackworms. :)

That two females to a male grouping is also used by killifish nuts who do not wish to see their females battered by over enthusiastic males. But too often while one female killifish is laying eggs with the male on gravel, plants, a spawning mop (or even a floating thermometer), the other female killie is following along, feasting upon those eggs. :(

I wouldn't be surprised if someone discovers that female guppies or killies could distinguish their eggs or fry from those of a neighbor. Certainly they can distinguish their species from fry of another species. So they need to be diverted with an even more attractive food source. I have also tried those worm jars with pairs and trios of killies. The results have been encouraging there too.