Given this, it's common for a 3-6 month "probation period" when starting a new role in which you can be dismissed fairly arbitrarily. This makes sense given the commitment taking on an employee represents IMO.
As a result, it's very common for people to lose their jobs within a few months of starting. Just as I imagine a lot of relationships end after a rather short period of time or then tend to persist a bit longer. Sometimes it's clear things just aren't going to work out.
She's right in that it's a bit of a rug pull to get continuously good feedback and then suddenly get terminated. There should ideally never be any surprises. I.e. you'd always usually tell the employee what will happen if x does/doesn't happen, get them to agree and then when it pans out that way, no-one is shocked. But like I said.. she's been there what, 4 months? Just move on.