I would expect HR to talk to her in this situation regardless? Someone who is extremely upset about something social her team and manager did, and then puts in for a week of PTO right afterward, needs to talk to HR - even if HR 100% has her back. (And if HR is 100% on her side, which they should be, hopefully that includes them having learned that they should be checking in with her before they do anything about her situation....)

It's hard to say from what's in the letter, and you could very well be right, but I would be just as worried if HR *wasn't* checking in at this point, because HR ought to be doing something about this, and they need to start by talking to her.