On a positive note, I commend your friend for his zeal and boldness in evangelism. The majority of Christians keep the good news to themselves while watching the world around them go up in flames. It takes a lot of courage to do what your friend does every Saturday, especially on a subway train. My hat goes off to him.
On the whole I agree that using a megaphone somewhere other than an open air setting is obnoxious. However, I'm not nearly as hard on him as others seem to be in this thread. If our sovereign God has determined to use your friend as the instrument to bring one of His elect to himself and the appointed time happens to be on a subway train on a Saturday then it doesn't matter whether your friend uses a megaphone or sits silently with a sign. Conversely, if some on that train God has sovereignly elected to pass over then those hearers are going to take offense regardless of the methods employed.
I've met too many Christians over the years, to include leaders in the church, who so badly want the world around them to accept and affirm them that they have not only softened their methods, but also their message. In a desire to be liked and loved by all we have packaged our evangelistic efforts into something that is non-threatening and safe to the unbelieving world around us. Rather than evangelize the stranger on the street we want to first build a relationship with that person showing them that we Christians really are a good and respectable people and once we've gained their trust we muster up the courage to tell them about their sin and misery apart from Christ two years later.
I went about my first 26 years of life having no idea that in my sin I was terminally ill and on a path to hell separated from God for eternity. I rode on trains, planes, and automobiles and nobody ever told me about Jesus....I played in parks, studied at school, walked to work, and dined with friends and nobody ever told me about Jesus. As I careened down the road of life swerving towards a cliff there was no man with a megaphone trying to get my attention. There are multitudes going about their busy lives who need to be woken from their slumber...all of whom are separated from God by their sin and desperately in need of the gospel message. Give me more men like your friend I pray! In hindsight, as a Christian, I don't care if God's appointed instrument was a boy with a bull horn or a lady with a lasso...I'm forever grateful to have been saved. Give me more men like your friend I pray!