Dear Rabbi Simon,
I attended a shiva house and there was a liberal service which may be charitably described as “interesting”.
It was more like a sermon where the person leading read out a mash-up of parts of Shema, jumping between Hebrew/English, and then parts of the Amidah which felt so far away from our version with things chopped out and added in.
Anyway, I just listened and paid my respects but was confused if I had an obligation to answer amen or not? Also, they did say Borchu, in Hebrew. Again- wasn’t sure whether to answer or not.
Thank you for your question. It is awkward to even be present at a heterodox, DIY, mash-up ceremony as you describe. Yet the mitzvah of comforting the bereaved is still applicable. (It is doubtful if one is even allowed to attend a service such as this in an actual Liberal synagogue, however.)
Turning to your specific question, the answer is that one is best advised not to respond amen or to borchu, kaddish, etc., in such a setting. These are all forms/expressions of affirmation, which are not appropriate when the prayer leader and the prayers themselves are theologically suspect.
May you only have occasion to attend simchas in the future.
Rabbi Rashi Simon