You can kibitz a player only when the host of the Table / Tournament allows kibitzing in their table settings.
The way to kibitz a single player is to RIGHT-CLICK on his username and select 'Kibitz'.
Using a mobile device hold your finger down on the username for a few seconds to get the options.
If a host of a Table / Tournament does not allow kibitzing at their table, you will see 'Kibitzing not allowed' (bottom left) when you click on a participating players' username.