I am 5ft4 and range between 7 and 7.5 stone (well not now I'm pregnant). It's just my build and I do think I'm quite light boned as know slimmer people that weigh more. My mums always been the same.
I've faced a lot of skinny shaming to be honest, so if someone looks healthy don't focus on their weight please. People wouldn't be able to say the same thing to an obese person, but asking a slimmer person about what clothes size they wear and how much they weigh is apparently ok.
The comments I got when I was suffering from HG and lost weight as a result actually were not fair and really upset me.
If this person looks unhealthy then yes it is classed as underweight according to BMI.
Sorry hit a nerve slightly! I'd just like more known about skinny shaming...