A tricky question. But luckily, one with a magical works-every-time answer.
You see, most of the time when someone asks you this, they aren't actually asking because they're considering it, they're asking because they've already decided to and want your opinion. The real question should be:
"I'm going on a diet. I want to know what you think of this."
The problem is, this puts you in a choke-hold. If you insist that they don't need to, it'll be futile and frustrating because they've already made a decision to and they're not going to change that - and if you agree that they do... well, you've basically just called them fat, which everybody knows is a very bad idea.
So, what is the perfect answer? The answer that has worked for me countless times and will probably work for you too:
"While I do not think you need to go on a diet, if you make the decision to, you have every right to do what you think is best for your body and I will support any decisions you make."
Because they'll either be expecting to be insulted or to be in a big long argument about whether they're fat or not, this'll come to quite a pleasant surprise, and they'll probably like you a bit more for it too.
One word of advice though - please be sincere about it. If you say you're going to support their decision - actually support it - you're not allowed to contest against it later because that invalidates everything you've just said - while this is most of the time a 'magic' answer, it isn't a get-out-of-jail-free card ;)
So! Second guide complete! I hope you found it enjoyable to read and useful for years to come - if you have any comments, questions or anything to add, feel free to leave something in the comments section below - also, if you haven't read the first guide yet, go do that too! :)
Until next time,