The 787 aft pressure bulkhead is manufactured by EADS-owned Premium Aerotec in Augsburg as a second-tier supplier (so also not directly sourced by Boeing).
These days, the supply chains for aerospace products are huge and involve multiple layers of suppliers buying from other suppliers themselves buying from other suppliers...while at the same time, the level of knowledge & skill required is very high, so the number of available suppliers is relatively low. In the end, you are bound to have some part of an aerospace group working for a "competitor".
All aerospace companies exist to make money by selling aerospace parts, by definition. As long as the supplier gets his money and the buyer gets good quality parts at a reasonable price, everyone should be happy, regardless of the name on the header of the in-voice. As for intellectual property, obviously the supplier will want to be careful, but on the other hand if (in this case) Boeing is buying the part instead of making it in-house, it's because Boeing is not really interested in acquiring and/or maintaining the competence and tools to do so. So it makes little sense for them to spend ressources to retro-engineer the parts.
Note that this cross-sourcing only happens indirectly and a different "levels of detail", through. These are subsidiaries of a group which owns one OEM that are supplying parts to the other OEM. I doubt that any OEM would directly supply a competitor, because for one thing the supplier would surely use his oversight rights to go have a direct peek at what the other guy is doing more generally. And more importantly, if Boeing found itself with no choice but to buy a product from Airbus (or vice-versa) because Airbus is the only supplier on the market, then Boeing management would have to explain why they left the monopoly to the competition instead of entering that market !
My goal as an engineer is to fill my soul with coffee and become immortal