First, you should know that since your results appeared first (as you stated in a comment), you don’t actually have a strong self-interest to try to make the follow-up results of the other researcher look bad. In a typical scenario, their publishing something that improves or generalizes your result will actually make your own work look better, not worse, including possibly if their improvement is not trivial (actually a trivial improvement may be slightly more embarrassing to you, for example if they discover that a trivial modification to one of your proofs ends up proving a much stronger result. But even then you’ll get the credit of being the first person who published an innovative new proof technique that was used to prove that strong result). So all things considered, it’s pretty likely that your incentives are actually aligned with those of the other researcher, and being as kind and charitable to them as possible (without compromising your integrity, of course) will be beneficial not only to them but also to you. And remember you don’t have to discuss every private thought you have: you may think the improvement is trivial, but at the end of the day, who are you to say? That’s just an opinion anyway. Consider not mentioning it and letting people draw their own conclusions.
What I wrote above pertained to the scenario of a correct improvement. Coming to the other possibility you mentioned, if you believe the work of the other researcher is wrong, then it’s wrong and you shouldn’t pretend otherwise or feign ignorance but simply be matter-of-fact about it - if you discuss the result, state that you believe it’s wrong along with your level of certainty that that’s the case, without gloating or schadenfreude. Don’t say you haven’t had the time to look at it if that’s not true. But you may consider simply not mentioning the result at all if its relevance to what you are discussing is not high.
Incidentally, not mentioning the result may also end up offending the author if they are attending your talk... it’s a tricky business, academia! ;-) Anyway, good luck.