To those who have answered this question 4 months ago, I am happy to say that my daughter and her friends have returned safely with truly great memories. Besides encountering some homeless men who have followed them, apparently, they felt totally safe.
Although we've kept in contact 24/7 throughout the whole trip I did feel more at ease after the 3rd day and needless to say that the duration of our calls gradually went from a burden of questions to "How are you girls doing? Having fun?"
I encourage giving our young graduates this kind of opportunity to cultivate independence, responsibility, self-defense, and instill that instinct of making the right choices.
Many thanks again for the help! =)