Ah, OK, I understood you feared they could never update to SP3 because they ignored it :D. In any case, I think it will not, but never found myself in a position to actually verify this, as SP3 is the first Service Pack Microsoft released I had the option to ignore, and opted for not doing it.
If the computer is up to date, the package won't be as big as a full update. And, unless there are reasons why is preferable to do an offline installation (say like, monthly usage caps), it won't eat their connection due to what I explained before. Did I get it now? :)