I've found great advice on apps, etc (not spy apps, but other kinds) by going to my phone service provider. I have walked in to Verizon and they are always willing to help me figure out my phone (I have a smart phone and dumb brain!). Maybe he could try that.
My other suggestion is that he doesn't try to do this secretly. I think this should be revealed in the counselor's office, and that he should be up front with his daughter. She needs to learn that trust is earned and that her dad won't go behind her back, but until she acts in an accountable manner (attending school, being where she says she is, being home by curfew, not drinking, etc), she loses a certain amount of freedom. When she's established that she's reliable and trustworthy, she gets some freedom back.
So the consequences fit the actions. Not being reliable and accountable = being monitored, being driven to places and picked up by dad rather than getting rides from friends. Parental controls are put on phones (again, have him ask his service provider - one of the people on our lines has gone over data usage and our provider taught me how to limit his access; there are lots of other parental controls). The wi-fi password is encrypted and only revealed after homework is done, and turned off at an acceptable hour on school nights. Parent is knowledgeable about secret apps (destructive and inappropriate apps that masquerade as calculators, for example; again, have him look these up). If the password isn't revealed to the parent, and if history is cleared before the parent sees it, the phone gets turned in for an old-fashioned flip phone.
Yes, she's not 4, she's 16. But that's not a legal adult. There's still time to help her become a productive and responsible citizen. Sounds like he's trying his best. Glad they're getting counseling. But tell him that the counselor will be good for him, too. When we've taken our daughter to counselors, we often get some correction ourselves, as her parents. We've gotten some good advice, and some tough love for ourselves. Parenting a kid with needs, whether they're emotional, intellectual, medical, often requires that the parents accept that they may need to do things a different way. Our son (who's older than our daughter) is a healthy, well-adjusted, independent, college-graduate with nothing but ordinary stuff in his life. Regular parenting, the logical stuff, worked for him. With our daughter, we had to throw out all the proverbial books and learn a whole different way of dealing with her, due to her many, many issues. We have definitely needed help. Just make sure your friend doesn't go into counseling thinking that the counselor will steer his daughter onto the right path and he can keep doing whatever he's been doing, or be a dad the way many dads get to be, the ones who's daughters are dealing with regular teen stuff like the occasional heartbreak, forgetting a homework assignment, arguing, etc. He may have to learn from the counselor right along with his daughter.