First of all, I think text messaging could really affect somebody's English proficiency if what he uses in sending text messages. For example, he is using shortcuts or he is used to type a message in a what-so-called "Jejemon" way of texting then his knowledge of the correct spelling of the words the he is using. Second factor is the length of tome or how often does he use his phone to send messages using the improper way of text messaging. If it is often then he would really get used to texting in that way and he may forget the proper way because he is already used in that way of text messaging. There is the tendency that he might be able to use it in writing some essays or other form of some requirements in English. In that way, your english proficiency may be ruined. As for people who don't use phones that much, I think their English proficiency wouldn't be that affected UNLESS we are affected by the third factor that could affect and ruin English proficiency that we have. This is when the person you are communicating through text messaging is using the improper way of texting where in you are reading text messages that are typed using shortcuts. In that way you'll also get used in that way of texting.
For me I'm not using my phone that much and I'm also not used into typing messages in a shortcut way. I'm not typing in a shortcut way because I already experienced writing the words in a shortcut way too. That's the reason why I stopped typing in shortcuts not only in text messaging but also in chatting. The bottom line here is, your English proficiency could only be affected by texting if you would us the improper way of typing words of your messages.
[Blog 3.3-Texting]