Yep. As long as she sticks with that phone she should be fine.
Not sure what will happen when she wants to upgrade though yet, but I suspect it will be like other grandfathered plans. it should work fine on 3g phones, but won't work on LTE devices (and possibly the 3g iphone)
I have a BB Bold, which I set to wifi both at home and at the office. As a result, I use very little data over the phone connection. Even when I didn't set it on wifi, I still rarely consumed more than 30 MB during any month. The 150 MB plan is more than adequate for my usage. In fact, I wonder how many BB owners ever surpass 150 MB in a month.