I would definitely NOT allow the friend to walk them. You have a lot of valid concerns and if you're not 100% confident in this person, I'd just say thanks but no thanks. Having the yard to run and play in seems like plenty of space to me.
I lost one of my babies to a car accident; so I'm probably more paranoid than most dogs owners about what can happen while they're outside. But I say it's always better to err on the side of LIFE.
Have a great time in Hawaii! Be sure to take lots of pictures!