Hi Jules,
I'll second Vanderhoven's expressed view. Here and here are posts (on another site) I made on the literary structure of that section of Matthew that also point to a first century Jewish generation being referred to at Mt 24:34. "This generation" of Mt 24:34 forms an inclusio with "this generation" of Mt 23:36.
I think Vanderhoven also clued in on translation issues: "All the tribes of the earth" could just as easily be rendered, "all the tribes of the land." But translators often believe just as your elder, that the phrase is referring to a future event, not the first century. Same with the phrase, "then they will see the sign of the Son of Man in heaven. (See Greek verse order here.)
The "sign" is of "the Son of Man in heaven." This is an allusion to Dan 7:13. Just a few days later Jesus associated the accusation that he would destroy the temple with this allusion to Daniel 7:13. (Mt 26:59-64) The "sign" was the destruction of the temple and city, which gave evidence that Jesus was also who he said he was and had the ruling authority to make the fulfillment come true. To the religious leaders back then, the destruction of the temple and city of Jerusalem was impossible since they were God's people. To even suggest such a thing was next to blasphemy.
In reality, your elder, by saying Mt 24:29-31 did not have a first century fulfillment, is expressing a lack of faith in Jesus. Jesus said, "Most truly I say to you, this generation will not pass away until all these things occur." The "most truly" (or "truly truly") is emphatic that "these things" would occur within that "generation." Even if the elder can't see how Mt 24:29-31 had a first century fulfillment, he should have the faith that there must have been some fulfillment.
Your elder's lack of faith is rooted in the WT's lack of faith that Jesus began ruling after he returned to heaven in the first century. See here for links to scriptural indications that this happened after Jesus' resurrection. The WT maintains that this did not happen until 1914.
On the 'trumpet blast' referred to at Mt 24:31, see here (and about two thirds of the way down in the post where the 1st trumpet in Rev 8:7 is referred to).
On Mt 24:29 and its parallel in Luke 21:25-26, see here in connection with the verses foretelling the Roman Civil War of 68-69 AD. Of interest is the possibility that Matthew's account was more for Jewish disciples, whereas, the gentile Luke, writing to the gentile Theophilus, gives much more detail (metaphorical, to be sure) about the Roman Civil War. Thus, Matthew 24:29 has only the cryptic reference to Isa 13:10. But Luke elaborates on the effects on "the inhabited earth" (Greek oikoumené) which in Luke's writings is almost always used in reference to the Roman empire. (Lu 2:1; Acts 11:28; 17:6 19:27; 24:5)