The IPv4 addresses reserved for private LAN use are:
10.x.x.x
172.16.x.x - 172.31.x.x
192.168.x.x
https://en.wikipedia.org/wiki/Private_network
Using 50.50.x.x for your LAN will work, unless you try to access a server on the Internet (like a website) with the same IP address. Then your router will send your packets to the local network instead of the Internet. The above private network IP addresses were reserved specifically to prevent this problem.
Port forwarding does not make your internal IP addresses visible to the Internet (and your friend). All it does is forward traffic sent to your router's public IP address to that specific port forwarded IP address on your private network. So this is not a reason to use 50.50.x.x.
Personally I use 10.x.x.x because it's easier to remember, and isn't used as much as 192.168.x.x so there's less chance of an IP address conflict when I use a VPN. Just be sure to set your subnet mask to match the range of IP addresses. A 0 in the subnet mask indicates the number can vary, a 255 indicates it's fixed. So a subnet mask of:
255.0.0.0 combined with 10.0.0.1 for your router's IP would allow all IPs up to 10.255.255.255.
255.255.0.0 combined with 10.11.0.1 would allow IPs up to 10.11.255.255.
255.255.255.0 combined with 10.11.12.1 would allow IPs up to 10.11.12.255.
(It's a bit more complex than this, but that should be good enough for 99.9% of people who just want to know what numbers to enter to get it working.)