If the other guinea were only killed off a month or so before the egg was laid, it can still be fertilized by a male guineafowl.
Otherwise, you might get a hybrid. Should be obvious enough whether it is or isn't anyway, they're not very ambiguous!
Good luck with that.
Best wishes.
Actually, looking at the sites of people who've had them, most guineafowl-chicken crosses were 'healthy free range'.
Others crossed in 'large, nice cages' too.
Haven't seen anyone get any hybrids from cramming them together in small cages.
You can't force them to breed either, lol. Don't...