I would say that the "engagement" "celebration would probably be seen as accepting the intended "marriage" as valid. One hashkafic discussion of this is in A People of Destiny
Note that "The Rav" referred to below is Joseph B. Soloveitchik who is called "The Rav" by those who learned at Yeshiva Rabbi Yitzchak Elchanan (Yeshiva University).
Against this background of destiny and identity many halachic social
constraints on Jewish-gentile relations can be understood and, in
paramount, the egregiousness of intermarriage stands out.
Intermarriage Rachaman litslan destroys Jewish identity and prevents
the rendezvous with Jewish destiny. Accordingly, the Rav zt"l was
absolutely adamant regarding the ban on attending an intermarriage.
Often we try to rationalize and justify attendance of such weddings.
We reason that if the Jewish partner comes from an assimilated
background and was never exposed to Yahadus, what right do we have to
judge him/her and boycott the wedding. Why should a ba'al teshuva
strain family relations by absenting himself/herself from a sibling's
wedding? After all, it is not the sibling's fault.
But, of course, that line of reasoning is fallacious. In refusing to
attend an intermarriage we are not judging another individual. Only
the Rebbono Shel Olam can pass judgment on one's liability, and to
what extent extenuating circumstances should be taken into account.
But although we refrain from judging individuals and affixing
individual liability in such cases, we can and must judge actions and
courses of action. Unquestionably, intermarriage is anathema because
it destroys Jewish identity and destiny. Accordingly, it is nothing
less than a chilul Hashem to be present at such a marriage. One can
not attend a wedding as a conscientious objector. By attending, one eo
ipso joins in celebrating. A Jew can not under any circumstances
celebrate the partial destruction of Jewish identity.
With this same compelling line of reasoning in mind, the Rav was also
equally adamant that subsequent to the wedding intermarried "couples"
must not be included in family gatherings or invited to family
semachos, and the like. Inviting the couple as such eo ipso
acknowledges and accepts their illicit marriage. Under no
circumstances may this acceptance be forthcoming. Let us not delude
ourselves into thinking that we would be simply maintaining relations
for purposes of kiruv. To the contrary, we are being m'sa'yai'a yedei
ovrei aveira, strengthening the hands of those living in sin and
creating a chilul Hashem.
May Hakadosh Baruch Hu save us from nisyonos and guide us all along
His path of Torah u'mitzvos.
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Similarly at Relatives Not Attending Intermarriage
However, since each case can be different, then a rav who is an expert in such matters should be consulted as to the best way to handle it.