A lot of people have trouble this.
The confusion is easily cleared up one simple question.
Are we talking about what terminates a single emphasis indicator or a double emphasis indicator.
A single emphasis indicator is terminated by a space, dash or slash.
A double emphasis indicator is NOT terminated by punctuation or composition signs.
This is not clearly explained in EBAE. You can find this referenced in the Literary Lesson Manual Section 15.1b.
Even though EBAEs Rule VII deals with Numbers and Roman Numbers, section 28e deals with more than numbers. There may not be a clear explanation but the example does shows the proper usage of a double emphasis indicator when a slash mark appears within the series of 4 or more words.
Unfortunately the rules and guidelines are not clear in several areas, so we are left to make the best decision we can, based on the information that we have available at the time. The best explanation I can offer you is that the new updates treat information on each side of the slash as a separate word or unit - so [u]his/hers[/u] is 2 words or units not one. With this in mind, it would stand to reason that, [u]Is this his/hers?[/u] Would be four words or units not three. And the rules for using single or double emphasis would apply.
I hope this helps