Help!?

Okay this is for my friend Aliya, who doesnt have a fanpop.

So she really likes this guy, Christian. Im real good friends with him. Hes one of my bestfriends. He admitted to me that he liked Aliya a few weeks ago. He told me to tell her, so I did. She was super happy. Then 2 weeks later, Christian told me that he really really likes Aliya. He cameover to my house & we had a talk about how much he likes her. They text eachother everyday. But his personality changes alot, and ive learned that from knowing him so long. Im not sure if hes bipolar, but yeah. One día he will be texting her for long time and will be all sweet and nice and charming. But then the siguiente día hes a total asshole to her. Then another día hes back to his normal charming self. Then goes back to his mean stage. Hes not like this with me, but it tends to happens to girls he likes. Im not sure why. Christian told me that hes been wanting to ask Aliya out, i told him go ahead, she will amor it . Then I told him if he acts like an culo to her like that she wont be your girlfriend for very long, cause I know her. But I really want them to go out. So my pregunta is, how can I like teach/tell him about how to act. Im confused, im not really sure what my pregunta is but can any of tu help please? It would mean alot. Thank you(: