Is it permissible to attend a non-frum event, such as a wedding or bar/bat mitzvah of a family member or friend?
This is just my guess on things from what I've seen.
There can be a lot of issues going on in each case, so it's worth taking an honest assessment of the full situation, and talking with a rabbi who's both knowledgeable and understanding. If both you and the event host demonstrate genuine caring and communication, that can help a lot of things too.
I think the overarching principles here, in descending priority, are:
A.) We can't violate strict Halacha.
B.) We can't give the message "oh Halacha is totally optional."
C.) Attending may fulfill a mitzva as it could be a form of respecting parents; kindness to those in need; or gratitude the hosts deserve. But be honest and careful; it's easy to rationalize anything into a mitzva if you want to.
D.) It's important to maintain our connections to non-observant Jews.
E.) It's important that non-observant Jews see that observant Jews are nice, caring, and "normal."
Otherwise, my guess is to respect C, D, and E above, so long as they don't violate A, B, and E. Examples that could potentially violate A, B, and E, and are thus more problematic:
- An event that will force you to violate Shabbat, kashrut, etc.
- An event whereby you are facilitating others' violation of Shabbat, kashrut, etc. The definition of "facilitation" is well-discussed in the rabbinic literature.
- The entire event is Halachically negative. E.g. an out-of-faith marriage.
- An event that will put you in a situation whereby keeping Shabbat, kashrut, etc. will make you (and therefore all Orthodox Jews, of course) look like an evil, heartless, stick-in-the-mud.
- A prayer service held in the sanctuary of a non-Orthodox synagogue. I've seen plenty of good Centrist Orthodox Jews attend such events; nonetheless, when I'd asked Rabbi Yehuda Henkin about a specific instance of this, he said, "I can't give you a psak without full understanding of all the pertinent details."
- A ritual that actively thumbs its nose at traditional Jewish thought and practice. It could be a Jew marrying a Jewess at a hotel, but if the officiating rabbi will use it as a big soapbox to attack Orthodoxy (in my experience, this is less of a problem in Conservative than in Reform/Reconstructionist), does your presence imply agreement? This could be less of a problem if you're Joe (or Judy) Orthodox in the middle of a big crowd, and more a problem if you're a high-visibility figure, a rabbi, and/or someone known as a Torah scholar.
If the hosts are making an effort to accommodate you, that can be a reason to attend (and not violate C, D, and E), but it has to be balanced with the above concerns.
One more note: Orthodox rabbis (even some fairly right-wing ones) are used to getting these questions. Otzar HaPoskim has the case of a BT woman attending her brother's wedding, at which she'll be expected to kiss him. (I guess it's just a huggy-kissy kind of family.) Answer: it's not so much Halachically prohibited as frowned upon, which gives some leniency. But better to explain the situation to him if you can.
This sounds like the sort of question that you really need to discuss with your Rabbi, since there are lots of different issues in play, and how they interact could vary a great deal, depending on the circumstances and on the people involved.
I'll enumerate some of the major issues that could come up. Note that I'm not saying that all of these are necessarily important issues in all situations.
- Entering a religiously liberal synagogue's sanctuary.
- Attending a religiously liberal congregation's services.
- Attending or participating in religiously liberal para-services at celebrations.
- Participating in a wedding conducted using religiously liberal procedures.
- Attending an event at which non-Kosher food will be served.
- Attending an event at which men and women will be dancing together.
- Maintaining a relationship with one's family and friends.
- Avoiding embarrassment to other people.
- Helping celebrants (especially a bride and groom) enjoy themselves.
- Sanctifying God's name by giving people a good impression of religious Jews.
The Gemara speaks harshly about those are invited to a Seudaat Mitzva (according to R' Mutzafi this means weddings or Beritot) and don't attent. However, HaRav Ovadia Yosef Shelit"a writes (Yabia Omer vol. 4 Yoreh Deah 19) that if there are going to be "Inshi Delo Maali" e.g. non-frum people it's better not to go. (I heard this from Rav Meir Eliyahu Shelit"a).