I mean, this is literally every big company IT department.
The best argument I could see that would lead to this succeeding is if someone at Facebook accepted the Snapchat ToS and it said "you can't use MITM to intercept our traffic". Otherwise, fair game, done billions of times a day. (Pin your certs friends.)
The arguable legal gray area is: If a user gives unlimited permission to do so, is decrypting another company's network traffic on device legal? I fall on the side of the fence that it's okay with consent because not even that other company has unlimited and exclusive ownership of a user's data or network packets preflight on their own device.
Btw at Meta/Facebook, even the data about employee devices has privacy filters on collected analytics because location, IP address, MAC address, serial number could be used maliciously by a rogue employee targeting other individual employees if there weren't protections (like Twitter was). Instead, almost all data at Meta is uniformly stored with hard privacy ACLs that, by default, prevent casual exploration. In other words, if I worked at Facebook and knew your Facebook accounts' graph FBID, I would be presented with minimal-to-no information about it and a "make a request for access for business use" dialog that is rarely granted by an appropriate manager or senior trusted person except for actual business needs. Even browsing my own Facebook account by FBID in the graph, I couldn't see all of the details, just a bunch of uninteresting housekeeping and general details. In general, they took at-rest storage of personal information seriously.