Under normal pressure...fooling around, they shouldn't burst. Right now we're doing 2Xdaily massage on mine to get them to settle down into the pocket and we're pressing pretty hard (the doctor went even harder). I don't think a rupture of the implant is as much of a concern as would be the valve failing.
I have heard of some women...serious lifters, who can pick up those cement balls and rest them against their chests without any problems. Did you ever play with the implant at the doctor's office? We did...they're tough!
Here's something I found though...
Failure properties of leaf valve inflatable saline breast implants...
I got 450's, but I was already a small D cup...this was kinda just to enhance what I already had. I like small boobs on other women though. If you like what she's got, reassure her that they look great and you could remind her that going big can be a pain to find cute bras and tops.
i asked my doc about this when i had my surgery done, and he told me about one of the girls in his office who had been in a car accident and hit the steering wheel at a pretty high impact, with no problems. i think you would have to hit them pretty hard to make one pop.