How can I enable chat in spectator mode?

Pressing Enter does not seem to open the chat and I couldn't find any option to enable it during the match.

When I spectate a friend's match, is there a way for me to chat with him sending private messages?

2 Answers

As specified elsewhere, there is no way to communicate with a person in another match without private messages or a clan channel. As well, you are spectating on a 3 minute delay; this is to prevent people from influencing the match by spectating (watching team A and warning a friend on team B, for example).

As you correctly noticed, currently, you can not open the chat dialog when spectating. Thus, you can not send text messages to your friend while spectating him.