I have a friend, let's call him Bob. I met Bob many years ago back when I was working in another country. We keep in touch over Facebook, occasionally we can have a small conversation.
Bob wants to learn English, so some time ago he started sending messages in English over his own language. He claims he does it for practice. However, Bob's English is not that great. I can understand what he is saying most of the time, however he does make noticeable errors in words choice and spelling.
Now, normally I would consider it rude to correct him, but he does claim to be doing this for practice.
How do I let him know he is making mistakes in a way that encourages him to continue to learn English?