Would welcome some unbiased opinions on this.
Person A - needs to lose weight. (This is not in dispute). They are seeking MH support for this due to a previous eating disorder. They have bought some dresses to slim into as an incentive as they can get too focused on the scales.
Person B - lives with A and says they are very concerned about person A and thinks they’re damaging their health. Likes to know daily how the diet is going, whether they’ve ‘given up’ already as they noticed them eating X or Y, tell them it’s a shame they’ve wasted a day if they’ve eaten something person B calls ‘grot food.’ They know this type of thing doesn’t help A but claims they’re too worried about A to not do these things. They see it as ‘helping’ and say they do it because they care.
Person B has today asked A if they were planning to send their new dresses back as they’re obviously not doing anything about losing weight and won’t fit into them. 
AIBU to think this is unsupportive? Person A and B live together, how can A handle B better as B isn’t listening to her when she says it’s not helping.