Well first off tell your friend to make sure his car is in proper condition. No brake lights out or anything like that. Second, do the speed limit, but no more than a few mph under the limit. If he does that his chances of getting pulled over and having to deal with the cops in the first place much much lower.
As long as they can't smell the weed, and he isn't doing anything stupid like smoking weed in the car on this trip, even if he does get pulled over I see no reason why the cops would want to search the vehicle anyway. If they do ask to search, he's gotta stick to his guns as say NO. No matter what they say, they are gonna get the drug dogs in blah blah if he doesn't consent to a search all they can do is give him his speeding ticket and drive away. Everyone is nervous when they get pulled over by a cop. That isn't an excuse to search somebody's car and a judge isn't gonna give a cop a warrant on a "hunch." He could pull the old, I smelled marijuana/alcohol, but when a crooked cop is involved you're going to jail regardless.
I'd ride with them both and not even give a crap. Then again I'm in my 30's and look like a straight edge dude. Don't exactly "stand out." I never once had a cop ask to search my vehicle just for shits and giggles even when I was young with long hair and looked like a stoner.
Mailing them is an option, but not to an old house where nobody lives. It has to be a real address. Mail carriers know their neighborhoods and if 2 packages suddenly appeared at a house that was vacant for months or years, THAT would raise some red flags. Honestly, I'd mail it to myself priority mail, so I would be home by the time it got there. If anything happened, again, I would stick to my guns and say I have no idea what that is or who the hell sent it to me. The thing is, anyone can send anything to anyone in the mail. If you don't admit to knowing about it there isn't much they can do.