Rebuking a Friend Who Violates Halacha

When my friend does something that I know is forbidden according to Halacha, but I do not know whether perhaps “one should not protest against someone who is lenient” or “he has an authority to rely on,” should I tell him that it is forbidden, or since I do not know for certain, should I avoid causing him embarrassment?

Shalom u-vrachah.

If he is someone who wants to observe Halacha, tell him pleasantly: “Check whether this is permitted; from what I found, there is an issue with it.”

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