If Romeo and Juliet were modern teenagers, they would be unlikely to turn to a friar and a nurse for help/advice. Who would their modern mentors be and why?

To some extent, this is an opinion based question, so the most important part of your answer is going to be the defense of your choices.  
I do tend to agree with the statement. I don’t think that Romeo and Juliet would be able to seek out a friar/monk for advice. That’s not exactly a popular or common job anymore; however, I do think that they easily could seek the advice of a religious official. That religious leader could be a pastor, youth pastor, rabbi, or priest. According to the Pew Research Center, more that 75% of Americans claim a religion of some kind.  
Based on that data, it still makes sense that Romeo and/or Juliet would feel comfortable talking to an adult that is a religious leader of some kind. Anecdotally, I frequently hear from my students that they are more comfortable discussing things with their teachers and youth pastors than they are comfortable discussing things with their own parents.  
The nurse is a bit more difficult to find a modern day equivalent of. She is the personal servant, guardian of Juliet, and that's not something that modern society has for somebody of Juliet's age. On the other hand, the nurse is much more to Juliet than a servant that gets clothes ready. The nurse is Juliet's friend and confidant. The two women are close enough that the nurse is comfortable telling dirty jokes and talking about Juliet's sex life with Juliet.  

Hie you to church. I must another way,To fetch a ladder, by the which your loveMust climb a bird's nest soon when it is dark.

In this manner, I see the nurse acting more like a worldly older sister than a caregiver. That's the modern day equivalent that I would pick. An older sister is somebody that would likely be living in the same house, would likely be close to Juliet, and would also likely take part in "girl talk" about boys and sex. A sister is also likely to support her sister first and parents second. Audiences see that in the nurse as well. She tries to defend Juliet from Capulet's anger in Act 3: "You are to blame, my lord, to rate her so."
That gets the nurse some verbal (and perhaps physical) abuse from Capulet, and that's when the nurse tells Juliet to marry Paris. I think a sister would do the same thing. A sister would stand up to a father to protect her sister, but that same sister might change after realizing that following dad's wishes is "safer" for everybody. 
https://www.pewforum.org/religious-landscape-study/