Mary
Uzzah and Delilah are right... the aspirin did not create your dad's ulcer. Stress causes ulcers, aspirin does not. ASA, like Uzzah mentioned, interferes with the blood clotting process, but it does not cause bleeding ulcers.
How much would you like to bet that the stress in your dad's life is JW related? How much time does your dad spend going to meetings, getting ready for meetings, reading the literature, going door-to-door, preparing talks and presentations, worrying about what The Friends™ are thinking of him and whether he's really doing everything he can, with Armageddon™ being Right Around The Corner™? Chronic stress is most definitely high on the list of suspects in developing an ulcer. Between stress related illness and depression, it's amazing that the average JW can function at all anymore.
Don't beat yourself up, sweetie. He's going to be OK, and maybe, just maybe, this will end up being a defining moment for your dad. Maybe it will make him think about how he was prepared to die a martyr's death for the WTS, and maybe he will come to resent the fact that the WTS would never make a similar sacrifice on his behalf. Maybe it will get him to realize just how dangerous the no blood doctrine really is. That may never have had a chance of happening without the chain of events that was set in motion with him taking the Robaxacet.
Take good care of yourself... you need to be in top form in order to help look after your dad.
We love ya.