R. Yitzchak Abadi has a responsum in which he was asked his question:

Shu"t Ohr Yitzchak 2:89:3

המ"ב (ס"ק א') הביא את החיי אדם שכתב דאם ראה את הקשת אין כדאי להגיד לחבירו דהוא מוציא דיבה ובספר ברית כהונה (מערכת ק') פליג על החיי אדם הנ"ל וכתב דאדרבה אנו שמחים בזה ועל זה אנו מברכים ע"ש איך ההלכה

 

תשובה אין לזה שום מקור שאסור לומר לחבירו ואיך אפשר לסמוך על מה שאין ידוע מקורו ובסמוך נביא ראיה מספר חסידים דלא כהחיי אדם אלא שיש להודיע לחבירו כדי שיברך

 

The Mishnah Berurah (Se'if Katan 1) brings the Chayei Adam who writes that if one sees the rainbow it is not proper to tell his friend, because he is spreading an evil report. And the book Brit Kehunah (Ma'areches Kof) disagrees with the aforementioned Chayei Adam and writes that on the contrary we are happy about this and this is what we are making a blessing on, see there. What is the [final] halachah?

 

Answer: There is no source at all that it is forbidden [for one] to say to his friend [that there is a rainbow]. And how can we rely on something whose source is unknown? And nearby we will bring a proof from the Sefer Chasidim not like the Chayei Adam; rather, one should inform his friend in order that he make the blessing.

The "nearby proof" is a reference to what he writes two questions later (2:89:5) where he notes that the Sefer Chasidim (807) discusses someone who was told during the repetition of Shemoneh Esrei that there was a rainbow and he went outside to make the blessing, and the Sefer Chasidim makes no mention of an issue of spreading an evil report by telling the guy about the rainbow.

R. Yitzchak Abadi has a responsum in which he was asked his question:

Shu"t Ohr Yitzchak 2:89:3

המ"ב (ס"ק א') הביא את החיי אדם שכתב דאם ראה את הקשת אין כדאי להגיד לחבירו דהוא מוציא דיבה ובספר ברית כהונה (מערכת ק') פליג על החיי אדם הנ"ל וכתב דאדרבה אנו שמחים בזה ועל זה אנו מברכים ע"ש איך ההלכה

תשובה אין לזה שום מקור שאסור לומר לחבירו ואיך אפשר לסמוך על מה שאין ידוע מקורו ובסמוך נביא ראיה מספר חסידים דלא כהחיי אדם אלא שיש להודיע לחבירו כדי שיברך

The Mishnah Berurah (Se'if Katan 1) brings the Chayei Adam who writes that if one sees the rainbow it is not proper to tell his friend, because he is spreading an evil report. And the book Brit Kehunah (Ma'areches Kof) disagrees with the aforementioned Chayei Adam and writes that on the contrary we are happy about this and this is what we are making a blessing on, see there. What is the [final] halachah?

Answer: There is no source at all that it is forbidden [for one] to say to his friend [that there is a rainbow]. And how can we rely on something whose source is unknown? And nearby we will bring a proof from the Sefer Chasidim not like the Chayei Adam; rather, one should inform his friend in order that he make the blessing.

The "nearby proof" is a reference to what he writes two questions later (2:89:5) where he notes that the Sefer Chasidim (807) discusses someone who was told during the repetition of Shemoneh Esrei that there was a rainbow and he went outside to make the blessing, and the Sefer Chasidim makes no mention of an issue of spreading an evil report by telling the guy about the rainbow.