Scenario:

Worker A asks boss to attend a workshop (worker a will pay for it) on a certain date and boss is fine with it-- it's for a few hours in the afternoon.
The date arrives and Worker B (coworker of Worker A) calls in that he isn't coming to work (ill relative in the hospital).

If Worker A goes to afternoon workshop boss will be alone- plus boss usually leaves 45 before the official end of the day because of early arrival in the morning (usually 1.5 hrs before 9am).

Should Worker A ask to attend workshop knowing that boss would probably flip out? Is boss insensitive if he/she doesn't make mention of the workshop? Should Worker A just drop the whole workshop thing?

Boss already agreed. I don't see the issue.

why would boss flip out? worker A set the date a while back. the scenario is that worker A asked to leave early and got permission. it's not the boss' business to find out why. it could be a doctor's appointment, picking someone up from the airport, or a bunch of other things. the boss doesn't necessarily get to decide whether worker A should be flexible. if worker A got prior permission to leave for that chunk of time, i see no reason to drop the workshop.

Maybe not flip out per se, but just put his foot down and say no.
Worker A doesn't take time off his/her hours (vacation, etc.) for attending workshop- because it's professionally related.

Assuming she asked for it prior to the date and gave enough advance notice and it has been paid for, A should go.

Assuming she asked for it prior to the date and gave enough advance notice and it has not yet been paid for, A should go, but if wants to be VERY (over and above necessary) nice she can tell boss 'I know that B is sick so if you don't want me to go to workshop, I can try and reschedule'.

Assuming she asked on the same day as workshop, B is sick, she can mention it in a way in which it is understood that her request may be denied, and only if he thinks it will be a quiet day and doesn't need the help.

Assuming she asked on the same day as workshop, B is sick, it's a very busy environment where 45 minutes with no worker can really be a HUGE hassle, she should drop it.

unless boss usually goes back on his word, no reason to believe he won't let worker go to workshop.
however, worker will never know unless worker asks.

Yes, permission was requested prior and received.

Yes, permission was requested prior and received. Cannot be rescheduled; it's a once annual workshop and professional development.

Unfortunately, Worker A is deemed a professional, Worker B is a para-professional. Boss usually doesn't get his 'hands dirty' with activities of A and B. A and B deal with the public and boss tries to maintain a 'behind the scenes' persona.

It's not completely that it's so busy, but boss leaves early (and won't stay late).
Ordinarily workshop is from 1-4:30pm. Worker A, when permission is ordinarily approved to attend such wkshps- 1 or 2x annually, usually doesn't return to work because of travel time, etc. it's not worth it as work ends at 5pm.

Today's wkshp is 12 blocks away and it's feasible (if Worker A wants to be considerate) to leave the wkshp early to allow boss to leave at regular scheduled time, i.e. leaving the wkshp about 30 mins early.

I say 'go'

so the boss will stay late to cover- isnt that part of being the boss?

Heh, not where Worker A works.

Worker A will probably ask to attend and then suggest to leave the wkshp early (appx 30 mins before the end) so that boss can leave at regular appointed time (45 mins) before the end of the work day.

If worker A is paying for the workshop out of pocket, no way should s/he leave early.

Unfortunately, even though the workshop costs ($10 + annual membership of $10, as it's a local branch of an international org)- it's irrelevant to the boss.