Is the laughter of worshipers at the action of someone who finishes their prayer before others, saying "First" as if in a race, considered mockery of prayer? And what are the consequences of that?

What happened is not mockery that expels one from the faith. The nature of disbelief-inducing mockery has been clarified in Fatwa No. 109383. We warn against obsessive whispering (waswasah) in this matter, as it leads to great evil. Your friend should be advised about the necessity of humility (khushu') in prayer and performing it properly, and that it is not a competition.