Yes, one can use a numeric (or longer alphanumeric) code for entry.
Or she can just be aware that one other particular person can unlock her phone.
There isn’t a fingerprint sensor to fall back on.
This, however, is akin to discovering that someone else happens to have the same numeric code. With 4 or 6 digits, vs tens of millions of users, odds are a couple people who know each other are likely to use the same code.
Key point here is it’s ONE particular pair of people who happen to look similar enough (to the device) that one can open the other’s phone. This is NOT a case of total security failure (as some will construe it). And this is a biometric convenience, not full-on data security; it’s akin to having a locked front door knowing full well a willful & motivated thief could easily break a window and climb in your house - to wit if you want serious security, use a passphrase > 6 characters, not merely face or finger biometrics.
Like one in a million. And that one just happens to be near you!
When I was a teen, my 1956 Chevy key just happened to unlock the doors on one out of four of my friends' Chevys, and vice versa. Not just that year, but several years around that manufacture. Chevy had a rap for cars being stolen and joy-rided. This thing with that Chinese woman's phone is a minor inconvenience, like you say she can use a numeric to unlock it. I often use a numeric when my finger won't quickly open my iPhone; not a problem.