
I suppose it is possible for the male who mated with her to have introduced an infection but that would be hard luck. Having said that, I'd NOT leave an in-season bitch with any male, neutered or not because quite honestly tensions are high enough when there's a bitch in season under the same roof.
In all the years we had entire bitches in our small show kennel, we never had a pyometra. We had one with juv.vaginitis when a puppy which took a while to clear (several switches of antibiotics until we had a swab, culture and identification done) but that was all. None of my bitches was taken off our property when in season, and each was kept quite away from all the other hounds, for the entire 21+ days.
I have no first-hand knowledge about being mated by a dog firing blanks and a false pregnancy. Again we had the occasional one doing this, but once retired, ALL our bitches were spayed. I'd suggest being mated like this would have no bearing on her going into a false pregnancy which I'd suggest wouldn't be evident until she had gone 'full term' (had she been mated by an entire male)
How long ago was the male who mated with her castrated because you do realise that sperm can live on for a while after castration!! She could be pregnant. I think I'd get her in to be scanned (and I don't normally suggest this!!!) just in case she IS pregnant.