If it's on the back of the shirt: Reach out discreetly and swipe it away at full speed. If it's on the front: Just reach out and swipe it away at full speed, and knowing my closest friends I'd probably do so while screaming, "THIS! IS! SPIDERRRRR!!!"
I'd scream and say "OMFG THERE IS A aranha ON YOUR SHIRT" then I'd grab the nearest blunt object and swipe it off their camisa following up por either beating it to death or leaving it alone and running away. ( depending on my mood )