Not everyone with bad social skills has ASD. Also people might go on and on for number of reasons (my husband has severe OCD-yes, diagnosed, and he at times goes on and on). I would not attempt to randomly diagnose people, neither others, nor yourself. It seems a trend on PC. People diagnose themselves and others left and right. I'd refrain from doing that

Saying that, I command you for trying to help. yes it's ok politely let the person know that it's time the change the subject. But unless they ask you, I wouldn't be advising them re their social skills especially if you are not very close. If they ask I'd suggest they seek professional help

Other than that it's hard to advice as we don't see or know that person.