If it were me, I’d try and find out if the person had relatives and express your concerns to them. Screenshots of their peculiar behaviour may be useful to serve as evidence to the relatives. Or, if that fails, maybe try and casually ask the person, “Is everything okay?” If it’s very early on in, say, Alzheimer’s, then they themselves may have concerns that their mind is failing them which they may have been previously reluctant to openly admit.