Vomit in the driveway? Good. Hopefully he swallowed the drip and vomited it out, getting rid of a tiny bit of the dose that way. I bet he felt sick and hot, went outside to get fresh air, and threw up. Not knowing your friend, it's hard to imagine what he would do after that. If he is an experienced psychonaut, the purging may have relieved his nausea, and he might even be enjoying a really, really, really intense trip. I hope this is the case.
Go to his house, keep peaking in through windows, try and catch a glimpse of every room. He must be somewhere. If not, wait in front of his house until he stumbles back there. Send out friends to places that he frequents.
I hope this turns out well.
/bs