My friend studied music in a university, graduated, and joined the state music band, which means he will always play in state events. However, He was disfellowshiped by the church on the grounds of playing instruments and singing with unbelievers. Is this right or wrong?
In addition to this, I have another friend and brother, who has an acapella group. They sing in denominational churches and get paid for it. They do acapella singing as a job or business where some members of the group are full-time singers. Have they committed any sin by singing at denominational churches?
There is nothing wrong with personally playing musical instruments. It is far too easy to take a statement, such as "God has not authorized the use of instrumental music in His worship," and overgeneralize it. I'm assuming the first friend is just playing with a band and that the music isn't for the worship of God. I must also assume that your friend's duties don't cause him to miss services and Bible classes. If all that is true, then it was wrong of the church there to have withdrawn from him.
Your second friend has a different problem. He is helping denominations that do not teach the truth. His stance tells everyone that to him what is taught doesn't make a difference (Galatians 1:6-10). He is also turning the worship of God into a way to make money. If it is like most of the denominations that I'm acquainted with, the group's singing is more entertainment for people to enjoy and not encouraging those present to join in worshiping God by singing as a congregation.