Pain yes, but breaking a silicon boob???
The AC above says that are hard/soft grades of paintballs - it would be more accurate to say how brittle the shell is and that can be affected by thickness, the consistency, temperature and other atmospheric conditions, and how they've been stored. Depending on the type of play people will want more or less breakability - too brittle and they break in the barrel, not enough and they bounce on the target rather than break when they hit. In general a rental paintball site (as opposed to a tournament team) will want tougher paint as the markers tend to be rougher on the balls; this has the added benefit of the punters having to fire more paint to shoot someone out and the site's profit is in the paint.
Paintballs do leave welts, and at close range can break the skin and even scar especially on hard areas like hands and arms. But breaking the skin on squidgy (e.g. fat) areas of the body is pretty rare even for close balls at high velocity. As a top-heavy lady on the cuddly side who used to play tournament paintball I can vouch for that - I've come away at the end of the day with plenty of bruising to the boobs, and welts with broken skin in other areas, but no broken skin on the boob area.
I see 3 options to what happened:
1) The Bulgarian fun-bags were faulty to start with
2) She damaged them in some other way NOT by being hit by a paintball or
3) The paintballs themselves were either of such low quality or had been stored so badly, that they should never have been used.
Mine's the one with an old Smart parts Impulse in the pocket