Do you still see ads even if your phone is offline? An uncle of mine refuses to use the free version of Astro File Manager, because he dislikes ads. 

I told him to just leave his 3G off (he only uses it on demand anyway) and he shouldn't see them anyway, as my expectation was that they couldn't be downloaded.

But is this really true? I tried it with Angry Birds and after turning off 3G, I no longer saw any ads, but is this possible?
Most ads require internet access.  If you remove the internet access, then it shouldn't load the ads (unless they are somehow cached?).

If he really, really, really wants to get rid of ads all together, you can install the AdFree app on rooted phones.  It will remove ads from the browser, from apps, etc.  Anything that is in its blacklist will refuse to load.

However, developers do try to make a living and often provide their apps for free at the cost of a little advertising.  You may also want to try and explain this to him and if he really likes the app he can support the developer further by purchasing the full (ad free) version.  No money, no developers.