Need Advice Hacking a Mobile Only Facebook User

Hi guys! I just found this site and this is my first post. I will start with a tl;dr because I suspect this might get long and I'm not sure if the title explains it well: I can't think up or google a way to get a facebook password of a mobile only user. I need to be subtle so the user doesn't realize I have access and I don't have access to the device.

Now for the long story. I'm studying CS and I'm the family tech guy. My Aunt has been having a lot of trouble with her teenage daughter since my uncle went abroad to work. She doesn't have the best relationship with her and is having trouble communicating with her. Recently she started seeing her with some rather shady people and is getting very suspicious of what she is doing when she leaves home. She asked me if I could help her at least keep tabs on her, but I'm unfamilliar with well "hacking". I only have experience with more classical computer languages such as C,C++,Java etc. I've read a ton on about accessing facebook accounts today and phishing is the only strategy I find viable atm, but I'm not sure how to use that on somebody who only uses messenger and instagram apps on their mobile phone. This is no long term solution, but it might help her stop my cousin from getting into immediate trouble.

Thank you all in advance, I really hope I can help her.