The way it is written means that she will be back in the office on Friday. However, I read it as her being out all week, including Friday. This is an error that is so common that most native speakers know what it means. Think of it this way, no one wants to work on Friday. So do you really think that she'll return on Friday?

Technically if it's 'until', then they should be in office on Friday. However, I'm inclined to think that it's just a mistake that native speakers make when they mean 'to' - as in Monday to Friday, which is the common expression.

Technically, When you use "until", it means that the person works on Fridays.

In my experience, by usage sadly ambiguous. If someone said that to me, I would have to inquire if they were working on Friday. Less ambiguous, °Monday thru Friday.°

i work from Monday to Friday.