I assume your friend isn't thin then? There's a lot of people who are not happy with their own body shape, but not ready to deal with it yet, who dont like other people losing weight/toning up/getting fit. It might not be something the conciously think, but there's an element if you try and then couldn't stick to it, then it's fine for them not to try as there's no point. Or if you are both eating the same foods, then they aren't having the 'bad' ones.
Your mistake might have been to tell her you were losing weight/trying to not eat 'bad' foods.
I'd have gone with just agreeing to a picnic, saying you didn't fancy fish and chips, you don't really like them, so will bring your own picnic food. Then that's about not wanting to eat something because you dno't want to eat it, not because you do want to eat it, but you are "better" at self control.
As you come out of lockdown, sticking to the weight loss will get harder. If you have alot of overweight friends, you do need to approach how you talk about eating and food carefully not to upset or offend anyone making different life /diet choices.
Try to make everythign about your choice/dislike, not about not eating certain foods, making different choices because you are tryign to control your weight. Kinder to say you don't fancy X than aren't going to have it because you are watching what you eat. You just really think Y sounds yummy, not that it's less carby/calories than Z. You aren't hungry/you have just had a snack/you're not feeling 100% so will skip that 'treat' thanks.
IF you have this a lot in the past with friends, then perhaps you need to look at how you talk about food and the impression you give when discussing diet. Other people might notice you have lost weight, never ever mention it or say you are on a diet. You just don't fancy x/just not hungry.