justalittlelemondrizzle hopefully you will feel confident now to tell your 'friend' it really isn't her business to comment on your children's weight, and especially not in front of them!
You may even feel justified in saying having looked into food and weight for children you are sure her children are overweight and you feel that is bad for them.
This may herald the end of the relationship but it may be for the best if she cannot stop saying things in front of your kids.
However, I do think that your older child is under weight, it really does depend if she is average height for her age.
My thoughts are based on the average heights in this chart...
www.onaverage.co.uk/body-averages/average-child-height
You can plot your children's weight on a BMI calculator here....
www.nhs.uk/livewell/childhealth6-15/Pages/Childhealth615home.aspx
If your older child is lower than what is considered a healthy BMI for a child of that age, there may be reasons for this and they may be totally healthy.
If you plot the children details - it asks age (date of birth), height, weight and sex - you will see if they come out below the healthy BMI band; and if you feel you want to talk to a doctor or nurse about it, then you may feel it is useful to do so. Don't let the fact that this woman is making poor choices for her kids stop you from investigating if you find one of your children is underweight.
Being over or under weight can be harmful. But having said this some people do not trust BMI indicators. And it may be that your child is a normal weight. I do just find it unusual they are the same weight, being one or two years apart in age.
I'm overweight and part of my problem is the mania of finishing up food and not wasting any. This is a really damaging message to give kids so it is very good your kids can eat when hungry and know when to stop. It really is a very valuable life lesson.
Good luck with your friend. And well done on encouraging healthy eating.