Why not just send them a SMS text message to their phone number? No data plan needed.
If you're going to be sending lots of text messages, then as listed above, there are other options.
Furthermore, I'm no BB expert, but if it has wifi, then internet apps, IM etc will work for free when hooked up to wifi.
Really, I think it's a little silly to get a BB with no data plan or no wifi at home/work.