I guess it depends. It sounds a very low weight to me. Is it an adult or a child? as they are measured differently.
If the person has lost weight without trying, that’s a concern. If they’ve lost weight intentionally, then they’ve gone too far, so yes a concern. If they are elderly and forgetting to eat, that’s a concern.
If they are an adult who has always been this size and appears fit and healthy, it’s not ideal but perhaps that’s their norm? Do they eat normally?
Apparently the BMI calculator is different for teenagers, if they are 16, they are just about in the healthy zone, assuming they have always been slim, but definitely monitor them.
I was 5’4” and weighed 7 stone 10 when I was early 20s. In pictures I look pretty skinny, but I wasn’t unhealthy. I’m more than 4 stone heavier now, plus I’ve lost an inch.