I checked online to see how other interested parties have come out on this question.
Grammar Girl votes for consistently treating the combined subject as plural:
If you feel you must use "and/or," my nonscientific survey of professional writing shows that you probably want to treat "and/or" as though it makes the subject plural. For example, Kelly's sentence would read
This message and/or attachments are confidential.
I must say, though, that I don't see how you could come to that conclusion and not insist that the construction requires an explicit "these" after the "and/or" conjunction:
This message and/or these attachments are confidential.
In contrast, Grammar Logs seems to focus on the fact that the final element in the combined "and/or" conjunction is or, meaning (as I read it) that the verb should match the number of the second element in the "and/or" pair, just as if the conjunction were simply or by itself:
Every grammar reference book I own warns against the use of this legalistic construction, and your quandary is another good reason to avoid its use. You can almost always substitute a simple "or" in its place or rewrite the sentence for greater clarity. I can't find an authoritative answer for you (if you insist on using the "and/or" construction), but I think I would treat this amorphous conjunction as a kind of or and base my choice on that.
Before proceeding further, I should note that the OP's "and/or" question presents us with two levels of complication: On one level, it poses the question of whether the plural noun before the "and/or" or the singular noun after the "and/or" should control the number of the following verb. But on a second level, it raises the issue of whether any pair of nouns sandwiching "and/or" should ever be followed by a singular verb.
The discussion at Word Reference Forums illustrates this second problem:
The president and/or the vice president makes the decision.
The proofreading function of my Word says "makes" should be "make".
But I think it should be "makes" because the verb must agree with "the vice president". (In this case, the president and the vice president are different people.) Am I wrong?
In other words, the WRF poster's Word program seeks to enforce a rule under which the presence of and in "and/or" automatically defines the subject as plural—even though the nouns on both sides of "and/or" are singular. This seems to be the rationale behind Grammar Girl's answer, too.
The most interesting response to this question in the comment string that follows points to the following discussion at Grammar-Monster.com of the conflicting rules governing either/or statements that include a plural element. The writer at Grammar-Monster identifies the first of these two approaches as the "standard convention":
If the pairings either/or (often the either is omitted) or neither/nor form part of the subject of a verb and at least one of the elements is plural, then the verb must be plural too.
Under this rule, " Either the budgies or the cat has to go" is incorrect. The second approach, according to Grammar-Monster, is the "proximity rule":
Not all grammar conventions agree with the ruling above. In fact, there is notable leniency on whether to use a plural or singular verb when one of the elements is plural. Under the proximity rule, the verb is governed by the element nearest to it.
Under this rule, " Either crumpets or cake are sufficient" is incorrect.
In the United States, the "proximity rule" is the more common style recommendation (which makes Grammar-Monster's "standard convention" the nonstandard convention in the U.S.). For instance, Words Into Type, Third Edition (1974) offers this guideline:
Plural and singular substantives joined by "or" or "nor." When a subject is composed of both plural and singular substantives joined by or or nor, the verb should agree with the nearer.
[Example] Others are trapped by the fear that their interests or their property is being threated.
The proximity rule is the rule I was taught, and it would certainly seem to point to a singular verb in the OP's sentence if that sentence were spelled out at full length as follows:
Both his co-workers and his boss or either his co-workers or his boss was at fault.
This awkward-sounding rendering of the sentence amounts to translating "X and/or Y" as "(X and Y) or (X or Y)"—that is, as a conjunctive "X and Y" and a disjunctive "X or Y" nested in a larger disjunctive statement that we might express as "Either (X and Y) or (X or Y) considered as two pairs of exclusive alternatives." But as some of the commenters at Word Reference Forums seem to suggest, it is possible to interpret "and/or" inclusively, as meaning "either or both of X and Y," in which case the "X and Y" would seem to command a plural verb, regardless of whether the specific match for the condition happens to come from "either of X and Y" or from "both of X and Y"—and regardless of whether X and Y are both singular, both plural, or one singular and one plural.
I have no idea which of the conflicting rules—"standard convention" or "proximity rule"—will win out as the dominant way to handle "and/or" in the future. The "and/or" conjunction seems to be in no danger of fading away; and if it is here to stay, publishers need to come up with a consistent approach to handling it if they want to avoid distracting readers from the actual content of their authors' prose whenever a problematic "X and/or Y [+ verb]" construction arises.
In the near term, though, I think that the "X and/or Y [+ verb]" construction is inescapably distracting—at least when the verb is number specific—and I wouldn't use it. To avoid that construction here, I would express the idea embedded in the OP's example along these lines:
Either his co-workers and his boss were jointly at fault, or one or the other of the two was entirely to blame.