We know The rainbow is a bad Omen. Is one allowed to tell a friend when he sees one, in order to allow him to make a Bracha or should he hold back as it is a bad sign?
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Per Mishna Brura (229:1), when one sees a rainbow he should not inform a friend about it, but rather make a bracha and keep it to himself.
As noted by Gershon, this is Mishnah Berurah citing the Chayei Adam. It seems that there might be room for manipulation according to other posekim. Thus:
For instance, the Aruch HaShulchan, which some people pasken like over the Mishnah Berurah, writes in the same siman, 229 that the prohibition is just staring, not simply looking. And he does not cite the Chayei Adam about not telling a friend, and so does not bring it down lehalachah. This might well make sense, since you would not be causing your friend to violate any issur, if you are just telling him so that he can make the beracha.
Of course, consult your local Orthodox rabbi, who will in all likelihood tell you to follow the Mishnah Berurah. :)
Rabbi Moshe Kohen of Gerba Tunisia writes (Bris Kehuna Ma'areches Kuf, Ois Daled):
I personally would say slightly different. If Hashem didn't want everyone to know about the rainbow, why show it just to a select few? Is it they who are the bad people and have to mend their ways while the others who don't see it themselves are all tsadikim and have no tshuva to do?