Perhaps you shouldn't tell Alice about Bob's sexual assault. Because your getting into a friend told a friend territory which is just not good. It is very weak evidence and often subject to being wrong.
This however is not to say Alice should not know about this if it is true, and it is not to say you should not act to let her know. After all it seems she is with a creep, and the consequences of this are going to at least be a very broken heart in the future to severe personal risk on the part of Alice.
Here is what I suggest, you engage Christine and Daniella to bring this message to Alice. It has all the benefit of what you want to achieve, letting Alice know what she is involved with, and not betraying Christine's trust. The message will be stronger, without any of the ambiguity of it just being a rumor. This will go a long way to help validate Alice's fearsany suspicion Alice may or may not have about Bob and help her possibly get out of harms way, then simply hearing what may be a rumor from you.
Both Daniella and Christine have some interest in helping you. Daniella while she does not want to press charges, still has some interest in seeing that Bob is stopped from doing this to other's, and is not totally afraid of Bob, else she would of kept her secret totally to herself. Christine cared enough to tell you about what she heard about Bob, she is concerned and wants to help. These friends are your allies here, bring them together to give this to Alice.