This is a tough one. I don't teach my kids to lie, but I also tell them that sometimes we don't share things that might hurt someone's feelings. I wouldn't tell my kid to lie and say they LOVE sloppy joe's, but to be respectful and thankful and eat it anyway. I make my kids eat whatever we have for supper, so its pretty common for them to eat stuff they don't like. And most people understand that kids are picky. I'd tell the lady, my kids have learned to eat what's put before them - or something like that. That way you're not lying and they don't have to worry about pleasing a picky kid. Because that was the reason for the lie, to make her feel better right? :)
About the track suit, you don't have to tell someone you exchanged the gift they gave you. You can thank them for the gift and leave it at that. If they ask you how did it fit or do you like to wear it, you could mention that though you really liked it - you had something so similar at home already so exchanged it for this other really cool outfit that you wear all the time. People usually are not super attached to what they bought you, they just want to know that you appreciate the gift they gave. You did and would've kept it had you not had a similar outfit at home. It would be rude to bring that up to them out of the blue, but if they pester you - its totally fine to let them know. What if it had been too small? You would've had to exchange it. People understand that stuff, and should not be offended by that. And those that are just need to lighten up! lol
It is confusing to kids to think its okay to lie sometimes. Pretty soon they will be using that to their advantage - and what is scary is that sometimes they don't even realize they're doing it. In sticky situations where you don't want to tell the truth - where it hurts someone's feelings - you can either avoid the question of outright say you'd rather not answer. Or learn to answer in a diplomatic way. Say someone asks if something makes them look fat. Obviously you wouldn't say, yep it does! But you can learn to say something like, I really like that color, but I'm not sure that's the most flattering fit, what about this instead... Whatever. I wouldn't want to walk out of the house in something unflattering because my friends were too afraid to tell me the truth.
Anyway, you get what I'm saying. There is a way to be honest without hurting someone else's feelings.
Best wishes!