husband cheating on wife

Hi, We have been married for the last 5 yrs. and it was a love marriage. Since last 3 months, my husband has been regularly calling a female colleague of his office, messaging her, calling her frequently during the night, chatting for 2-3 hours and even dropping her at home. When I confronted him, he denied having any affair, he just says that she is a very good friend. But I feel something is wrong because he has not been caring for me like when he used to do before, lies to me now and then and also frequently takes up a fight with me. He is also an alcoholic and a smoker but this has increased since the last 3 months and now we have been fighting daily because of this girl. when he is sober, he pleads guilty and assures me that he will not have any further contact with this girl. But this happens only for 2 days and on third day he again starts contacting her as usual. Things have now gone from bad to worse. This girl as I came to know is getting divorced on October. I love my husband and I want to save this marriage. Can you guys advice me on how to get rid of this girl as I am scared that her demands may increase after her divorce. Also, my husband in case he wants to get rid of me what pre action should I do so that he cannot escape easily. Last but not the least, I feel my husband is taking advice from one of his close friend who is a lawyer in Delhi and also on ways on how to cover up all his mistakes and get rid of me easily..