Cellular data: a simple way to beat cellular voice fees.

Yes, except SMS is not real-time (but almost) which may or may not be an advantage. Archiving of logs is scattered. The user does not have to be online. Protocl features differ, and there is the problem of which IM protocol to use. Defacto standard differ a lot per country, even community.

If your friend has Java ME on phone they can install IM client like Mig33. Usage may or may not be cheaper than SMS.

There used to be free HTTP/SMTP to SMS gateways, but they included some spam, and God knows what they did with the entered personal data. SMS to HTTP/SMTP gateway might be useful too, e.g. in case of data network down and/or emergency.