One of my Muslim friend's husband loved another lady and got married but did not divorce my friend. She is staying with her parents now. More over he drinks alcohol too much. But presently he is regretting what he has done and is calling his first wife back and doesn't want his second wife, should she return to him in this situation?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The person mentioned in your question should give up drinking alcohol whether it is little or much. Allah Says (interpretation of meaning): {O you who believe! Intoxicants (all kinds of alcoholic drinks), gambling, Al­Ansâb, and Al­Azlâm (arrows for seeking luck or decision) are an abomination of Shaitân's (Satan) handiwork. So avoid (strictly all) that (abomination) in order that you may be successful.}[5:90].
So, whoever wants success and blessing in this life should never drink alcohol. In addition, whenever he repents to Allah, gives up consuming alcohol, adheres to the rules of Shariah, becomes steadfast and performs the obligations of Shariah such as prayers, fasting then he will be considered a good husband.
Therefore, he may take back his wife and then the wife should not delay in living with her husband.
If the husband does not give up drinking alcohol and continues on his malpractice, we advise the wife not to return to her husband since in this situation he might be harsh with her and might force her to lose her religion.
Moreover, it is more likely that if she abstains from being with him he may then give up his bad practice and may repent towards Allah.
Allah knows best.