My friend invited me to his chasuna. I had not planned to go, but then it turned out that I didn’t feel well that day. May I call him afterwards to wish him mazal tov and mention that I couldn’t make the chasuna because I wasn’t feeling well? Perhaps, that would be a good idea because it will promote friendship between us, and furthermore, it will make the chosson feel good.
No, and the reason is because it is both geneivas da’as and being untruthful.
The issue of geneivas da’as is based on a Gemara (Chulin 94B) which says that one may not invite someone to eat with him if he knows that the prospective guest will not accept the offer. The Gemara is speaking in case the host would not have invited the person if he thought he would accept the offer. The reason it is forbidden is because the person is being given the impression that the host is a good friend and really wants him to come, when in fact the host is not such a good friend. This is the idea of geneivas da’as: giving false impressions.
There is a case of geneivas da’as that is mentioned in this Gemara (Chulin 94A) that is very similar to your situation. The Gemara states that one may not open a new barrel of wine for a guest if he sold the remainder to a merchant, since the guest will get the impression that his host was willing to forfeit the remainder of the barrel in order to honor him with a fresh barrel, when in truth he only did it because he had already sold the remainder.
In addition, if you state that you didn’t participate because you were indisposed you are not telling the truth, since you would not have attended in any case. This is derived from a Gemara (Yevamos 65B) that says there is a mitzvo to distort the truth in order to promote peace. The Gemara deduces this from Hashem’s reply to Shmuel. Hashem told Shmuel to go to Beis Lechem and anoint Dovid as king. Shmuel asked Hashem how can he do it since, if Shaul hears what transpired, he will kill him. Hashem replied that Shmuel should take a calf and announce that he has come to sacrifice the calf. The Maharsha asks that in this case Shmuel wasn’t being untruthful since he actually did sacrifice the calf.
The Maharsha replies that even though it was true that Shmuel brought a sacrifice, nevertheless, since this was not the real purpose of his visit, it is considered untruthful and therefore we can learn from here that one may distort the truth to promote peace.
Similarly, in your situation, since your illness was not the cause that you didn’t attend you would be untruthful to say that it was the cause. In the sefer Borchi Nafshi (Toldos), Rav Eliashev’s son-in-law writes how this happened once with Rav Eliashev zatsal and he told his family not to say that the reason he didn’t attend was because he wasn’t well when he wasn’t planning to go anyway.