Battle.net keeps showing my friend as online one minute, and the next offline for 20 hours or a day, or other random time. This has been going on since the last couple of updates to battle.net. It is random. This morning it showed my friend online. Less than an hour later it showed her as offline for 20 hours. Now she’s online again. This is all in less than a two hour span.
I am not sure if this is the case with your friend, but in the mornings I boot up Battle.Net this way I can catch updates, or have it ready if I decide to play.
All day long it shows me going online and offline no matter if I log into the game or not, it does the same for my friends, who are sometimes catching updates, like I am, or just have their mobile running off and on all day.
This is a different thing entirely. Today I have been in Battle.net all day, and it has alternately shown my friend online, then offline for 20 hours, then online, then offline for 14 hours. It is inconsistent in tracking when friends are online or not and for how long they are not. This is a bug.