I'm not sure you'll be able to win that argument, because I don't think it is impossible if you are allowed to use an arbitrary unknown energy source. To travel between stars in a few decades does not require any significant acceleration, if you can sustain it the whole time. The formula there, for approximately unrelativistic speeds, is that the acceleration needed to go a certain distance in a certain time is of order the distance divided by the square of the desired time (you can see that just from the units of acceleration.) If the distance is 10 LY, let's say, that's about 10 to the 17 m. If you wanted to get there in 30 years, let's say, that is about 10 to the 9 seconds. The required acceleration is then of order 0.1 m/s^2, which is only 1% of what we would call 1 "g". The hard part would be to maintain that acceleration for decades. Or, you could increase the acceleration and shorten the trip, though to get it less than a few years (in the traveler's reference frame) requires relativistic speeds and that requires a lot of energy. The aliens would need to very efficiently convert a significant fraction of the mass of their ship into energy, and we currently know of no way to do this. Is it theoretically impossible? I don't think so. But it can't be easy, or our solar system would be lousy with alien visitors.

In other words, I think the argument proceeds in the opposite direction that you are pushing: It's not that we know aliens aren't here because it's so difficult, we know (or suspect) it's so difficult because aliens aren't here. Of course, if someone thinks aliens are here, that argument won't convince, but then, probably they are not very conducive to listening to reason anyway!