You could change the default file permissions to 0666 with uname's set for each user. But that, especially with free access across the LAN has serious security issues. One compromised account on one machine compromises every single file on every machine. Linux is designed from the ground up to prevent exactly what you ask for from happening. Modernization needs to include understanding that a 1990s concept of security is not enough. I monitor my (standalone) firewall. I see port scans go by every few seconds. I think of nearly every one as an attempted intrusion, because there is no legitimate reason to scan every port on an IP address, ever, except intrusion, or intrusion prevention checking. And I'm on a dynamic IP, not static. You will be doing your friend a favor if you don't do what he asks--sooner or later.
A solution to file sharing that I use is a single central machine running a file server on nfs v.4. That disk partition is mounted noexec and nosuid (and probably some other restrictions as well). All users have an account of type nologin on that machine. All users belong to the "users" group on that machine. All other machines have that partition mounted locally via nfs on /home/share. You also have to be sure that all numerical userIDs and groupIDs are identical on all machines for this to work.
The end result is that anyone can read/write to the same /home/share partition. The files there will show their userID, but be in the "users" group. Default file permissions are 0664, so anyone can also read/edit other's files. Directories are owned by root:root or another group "higher" than users so that some directories can be privately shared only for owners, bookkeeper, etc.
All work that is not personal is done directly into the nfs share, across the network. We see no noticeable latency on read or write above when a file is read/written locally. And our server is still spinning rust, and the network only 100 Mbps
We've used this system for years. Loss of connection to the nfs is rare, and takes only a remote ssh and mount command to restore. (or a local reboot, but that's overkill) It is necessary when power is lost that the server boot up first. A formal backup procedure is required, but that should be the case anyway.
Just my 2 cents.
PS: don't let the backup comment slide. I use a triple mirror disk array on the share, for performance and redundancy. It is snapshotted every night, and duplicated offsite every week. Another advantage of a file server setup is the same disk array also receives nightly rsync copies of every home directory on the network. When I do once a week offsite backups, I only have one machine and one drive array to backup--so that is much easier, and guaranteed complete. I do provide each user with a subdirectory in their home called NoBackup, which the backup script ignores. They can put large easily replaced, disk-eating media files there. Or you could make the script ignore certain file extensions.