Whether or not we should "pray with" them... and whether or not we should participate in these events... are different questions. Rev. Morris didn't pray. From my view of what he said, his Trinitarian blessing actually stuck out like a sore thumb against all the generic nonsense everyone else was speaking.
It gets to be an odd question, though. Should I decline if I am asked to offer a prayer before a meal at a community dinner, because some who are present might not be Christians? IF this is not a WORSHIP SERVICE (and I don't think it is), offering a prayer in this context is no different... but these events are sort of in a grey area... not really worship, not the same as a community event, etc...
My general rule of thumb... if such a tragedy were to happen in my community and I were asked to join... would be, to ask to be given the chance to instead read a word of Scripture or speak a meditation upon Scripture, rather than to pray.
That said -- I still don't think these events count as syncretistic worship services. They are community events, for the purpose of hearing various words of comfort, in a pluralistic world. Again, the advantage of being in a pluralistic world is that we actually get an invitation... we can present a clear Gospel message in these forums... as Paul did in Acts 17... without trying to correct an entire pluralistic culture in a single statement. Correcting pluralism doesn't happen in one statement, it happens as you engage people with a message that IS exclusive... and guide them through the struggle over time.