Quick assessment of the situation. You can get text from BB1 but not from BB2 with both being the same model. The issue might be with your friend's BB or the sim card. It could also be yours. I'm assuming that your friend is a guy to make it easier.
I've thought of some quick tests that you can try. Not sure how effective the tests are, but it's something to start
1) Testing your friend's BB and sim card
1a) Have your friend put his sim card in another phone to test the sim card. Do you get his text instead of a 'null'? If yes, then we can elimnate sim card and say issue might be with his phone move to test 2.
1b) Put another friend's SIM card into your friend's BB and have him text you. Do you get 'null'? If yes, it is his BB. If no then we can do the same tests for your phone
2) If both of the above comes back good (no 'null') then we will assume that the issue is your Atrix or your sim card.
2a) Put your sim card into another phone and have your friend text you from his BB. If you're getting the text, then the culprit is your Atrix.
2b) Put another sim card into the Atrix and have your friend text that number. If you're getting Null, we can say that there's something that's affecting your Atrix.
If the Atrix is at fault, not sure what you can do other than a factory reset? Since this will take out all the variables of you downloading an app that might be causing this weird issue. It's bothersome but you might want to try it. Back up everything on your phone before you reset it. Have your friend text you and see if you're getting null.
If you don't want to do a factory reset and still text him. You can try Whatapps. It's like bbm/sms between phones. Have your friend download it

Both of you need data as it uses data to send message