Ical to Outlook issue

I have someone using an Iphone 7.  She has an Icloud calendar.  She had sent herself an invitation to her work email using Outlook. (connected to an Exchange server)  I don't use an Iphone so I am unsure of what it should be doing.   At first she said she couldn't sync back and forth.  Now she is saying she used to be able to but it just quit, but that the problem is with Outlook and not her phone..  She said she has a way to read it now.  Here's what she emailed me:
"Outlook isn’t letting me join it – it doesn’t work. It’s not my phone-  it’s our computer/server that has the issue. We somehow get it to be “Fixed” for a while- and then it just stops working all of a sudden. "
What she is said in a previous email is this:
This is how I “join” the calendar (see below) – officially (so it gives me updates as I make them) – but this stopped working so I tried to redo it and now it won’t work.
I also found a way to get a read only copy which is what I’m using now, but that way won’t update and so it’s just an old read-only version and isn’t helpful.

So I am a little out of the loop on this.  I can't tell her something won't work since she has told me it has before.

Any help would be appreciated.
rpateyAsked:
Who is Participating?
 
Tim LapinComputer Consultant (Desktop analyst)Commented:
Hi rpatey,

I don't see any attachment or photo, as implied by the sentence ("This is how I join ... (see below)").  Was there supposed to be an image?

Regardless, the best and simplest way to see her calendar is to add her account directly on the phone and NOT via an invitation.  It is done via the Settings app on the phone.  See the following image link as an example of how it looks in iOS 11.x:

Speaking of which, we could use some data:
-  Version of Exchange server (including patches).  Some versions have introduced problems for 3rd party devices such as the various iThings.
-  Version of iOS on the phone.  Same reasons.
-  Steps she's taken.
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