Since SMS requires a cell signal, you can't do it directly with a wi-fi only Nexus 7. You could install Google Voice, but the phone number it's linked to would need a texting plan (see this
support page) and the N7 would need to have a wi-fi connection in order to use it. Do you have a landline? Check with them--a lot of landline services now come with free texting (like my Comcast voice service), and you could link Google Voice to that number.
Other apps like WhatsApp require both parties to download the app, so that's not as clean a solution for you.
EDIT: I spoke too soon--I don't think Google Voice can be installed on the Nexus 7. I'll have to look into this a little more. Sorry for the bad info!