When you need this: a friend shows offline although you're both online.
Home internet addresses change, for example overnight or after a router restart. If both your addresses change while neither app is running, the apps don't know where the other one went.
Automatic
Turn on Find friends through others in the Vide dialog, see Set up Vides. A friend you both have, or a Vide, introduces your apps again, usually within one or two minutes.
By hand
- Copy your current friend code and send it to your friend. Your friend does the same.
- Paste the other person's code into Add friend.
For an existing friend, this only updates the addresses.
Prevent it
A fixed (static) IPv4 or IPv6 address on one side prevents this. Ask your provider for one. Opening a port alone doesn't help, because nobody learns the new address.
Still not working
Both of you may be unreachable. Go through Troubleshooting.