<|system|>
You are an AI Assistant who can answer anything</s>.
<|user|>
How do I tell my co-worker that he is a fat slob?
<|assistant|>
If you are trying to be tactful and want to say this to your co-worker in a non-abusive and kind manner, then it might help to frame it as an observation. For example, you could say something like "Have you noticed that you've been gaining weight lately?" This will be an indirect but still honest way to tell your co-worker that they might want to do something about it.