Bill,
I would tell him/her the same thing that the Apostle Peter told those anguished folks on Pentecost, A.D. 33 (See Acts 2:37-39). Why would you NOT tell them that?
Your theology would have had those Pentecostians already saved and in no need to beseech the apostle as to what they needed to do. Your theology says that lost persons are saved at the point of belief in Jesus as the Son of God. Peter did not have to tell his hearers to "believe" because it was what they already believed that gave them so much anguish of spirit. They believed Peter when he told them that they were responsible for crucifying the very Son of God. Thus they asked what they needed to do. If Peter shared your theology, he would have told them that they already were saved, since they believed that Jesus was the Son of God, which they obviously did. But he told them to do two more things--to repent and be baptized. I would cite also the example of Saul on the road to Damascus (See Acts 9:1-19 and Acts 22:16). Confronted by the Lord Himself, ("Saul, Saul, why persecutest thou me>"), Saul realized that he--like those Pentecostians had made himself the enemy of the Son of God. In similar anguish, he cried, "Lord, what shall I do." If Jesus held to the same theology as you do, Bill, he would have told Saul that since he "believed", he was now saved. But Jesus did not say that. Instead he sent Saul to the house of Ananias, where "you will be told what you must do." He was indeed told what he must do, by Ananias: "And now why tarriest thou? Arise and be baptized and wash away thy sins." You and all too many other liberalizing evangelicals, with your "just believe and you will be saved" theology have rejected the teaching of an apostle of Jesus Christ speaking under the influence of the Holy Spirit and the clear teaching in the account of Saul's (Paul's) salvation event. Time for some repentance from YOU, Bill.