I was curious if your friend and his church had a reason to gather around this particular verse. Sometimes a fellowship group may gather around a common idea as a response to something that had occurred or was said. Perhaps having this detail may make it easier to speak more directly to the heart of this church.
Jesus taught that we must seek those spiritual things to enter the kingdom of God, that what is born of Spirit is spirit (Jn. 3:5,6). When John the Baptist was doing his best to lead all by the waterside for baptism as a pledge for repentance of their sins, he acknowledged that it was the greater One who would come with what the people really needed: a baptism with the Holy Spirit and fire (Lk. 3:16).
In this passage of Acts, the disciples had just been filled with the Holy Spirit and were miraculously speaking in other tongues recognized by the people there for Pentecost (Acts 2:1-11). The people thought this could not be and that they were just babbling after too much wine (v12). After Peter reminded them that it was a bit early in the day for that (v15), he told the people about what would happen in those last days:
Acts 2:17-18 NASB
 ‘And IT SHALL BE IN THE LAST DAYS,’ God says, 'THAT I WILL POUR FORTH OF MY SPIRIT ON ALL MANKIND; AND YOUR SONS AND YOUR DAUGHTERS SHALL PROPHESY, AND YOUR YOUNG MEN SHALL SEE VISIONS, AND YOUR OLD MEN SHALL DREAM DREAMS;  EVEN ON MY BONDSLAVES, BOTH MEN AND WOMEN, I WILL IN THOSE DAYS POUR FORTH OF MY SPIRIT And they shall prophesy.
This was a fulfilling of what the prophets of old had foretold, and God’s promise was being fulfilled through Jesus Christ (v22). Not only were they missing this event, they had put to death the very Messiah who had came to save them (v23). But this was not to be the end. It was foretold (vv25-28), and He lived so that all may be saved, which these disciples were witnesses to (vv32-33).
When the people heard this and realized that they had partaken in the mob that put the Messiah to death, they were “pierced to the heart”, remorseful, grief stricken (v37a). They were ready to repent from the heart and wondered what they should do (v37b). To which Peter replied:
Acts 2:38-42 NASB
 […] “Repent, and each of you be baptized in the name of Jesus Christ for the forgiveness of your sins; and you will receive the gift of the Holy Spirit.  For the promise is for you and your children and for all who are far off, as many as the Lord our God will call to Himself.”  And with many other words he solemnly testified and kept on exhorting them, saying, “Be saved from this perverse generation!”  So then, those who had received his word were baptized; and that day there were added about three thousand souls.  They were continually devoting themselves to the apostles’ teaching and to fellowship, to the breaking of bread and to prayer.
It was a beautiful ending to that event which continues to play out each time a new believer comes to the same crossroads in their heart, repents of their own rebelliousness and asks what they may do. Peter explained this baptism further in one of his letters:
1 Peter 3:21-22 NASB
 Corresponding to that, baptism now saves you-not the removal of dirt from the flesh, but an appeal to God for a good conscience-through the resurrection of Jesus Christ,  who is at the right hand of God, having gone into heaven, after angels and authorities and powers had been subjected to Him.
Although baptism is closely related to the immersion of the Holy Spirit over the person, it is also a physical pledge to God and a symbol that the individual has accepted the death and resurrection of the only one who can save: Jesus Christ. Following in His likeness as an obedient disciple, we too take part in physical baptism to express the spiritual thing that has taken place.
I hope that helps some. There may be more eloquent arguments on the technicalities of baptism and symbolism, and we could ask legalistically what is someone’s salvation really worth if they won’t accept baptism. But rather than go into those details, I hope these words of Peter and the focus of the outward physical sign and pledge of what is taking place on the inside gives both validity to the importance of baptism, as well as the greater heart and spirit matter that takes place within regardless if there happens to be water around. The Spirit does the spiritual work of salvation, the water part is step of dedication in that new life that has already begun.