Actually, It
is possible to do this, sorry! In this reply by Jason (
viewtopic.php?f=27&t=2396) going into the Settings icon and tapping "Hide Mobile Chat Tab" will also show the user as offline, I did a test of this, and after a few refreshes, the user did go offline, and manually removing the "ac_hide_mobile" cookie shows the mobile chat tab again and the user appeared online again.
The only problem now is to find a way for the user to show the mobile chat tab / go online again, in my case, I added a checkbox in the user settings area of my website which when unchecked (if the user has the mobile chat hidden, detected by its cookie) will delete the cookie that controls the hiding of the mobile chat tab.