I doubt that you were the reason, Winter Wind.
Suicide isn't something you 'suddenly decide on', or so I have experienced. I have little doubt she has thought about it before. Even if your boyfriend could have kept her from it at that specific moment, possibly, she would likely have done so another time. In the end, it was her decision, not one you forced her to take.

As for what you can do for him... let him know you're there, but do not push him. Make sure he knows you're there if he wants to talk or scream or cry or pray, but don't keep urging him to talk about it or let out his emotion. Everyone reacts to the death of a loved person in a different way.
If he does not want contact for a moment, or shuts you out, that may be his way of dealing with it. Just make sure you are there for him... but let him make his own decisions in how he wants to deal with it.

The fact that you are afraid you might be taking advantage of him, that want to be there for him, that you think about if you have the right to do so proves to me that you are not taking advantage of him. You care about him a lot.