Gather proof of her adultery and than file divorce. Once you will have proof of her external affair, her cases if any filed will fruit nothing.

as the span of marriage is really short,

ask your brother to try to settle the matter amicably with his wife involving elders/relatives/friends etc and told her not to have contact with any male friend, 

before initiating any talk take backup of some whatsapp chat/ call recording etc, which will help you in future litigation,

he can file contested divorce on the grounds of mental cruelty, adultery (Whatsapp chats, pics) but it will take around 3-5 years,

also once he file contested divorce she will surely file/claim

498a complaint 

Domestic Violence case

maintenance,

better to settle the matter with the help of close persons

Gather evidence of her extra marital affair if any 

2) file for divorce on expiry of one year of marriage on grounds of mental cruelty 

3) sex chats with other guys amounts to mental cruelty 

4) if wife files false dowry harassment case apply for and obtain Anticipatory bail from sessions court 

Keep tabs on the girl 

gather evidence against her 

audio , video recordings would help you in making out a case 

collect as many evidence as you can, 

call/video recordings, screenshots, chatting are admissible in court to prove her adultery which is a valid ground for divorce

Hi, if you have enough evidence of her having relationship with another man ,it is advisable to file for divorce petition in court on the grounds of adultery 

Dear Sir,

Your brother can ask her parents and request her to come to your brother and if she does not agree then file a petition on the ground of restitution of conjugal rights through a local advocate.

Yes your brother can file Petition for RCR, in the court where your brother and his wife resided after marriage. Also along-with your brother  matter in the concerned court, the matter should be taken for mediation. RCR petition along-with mediation will help your brother more in convincing his wife to move back to her matrimonial home.
Also your brother  have to make sure that she has no valid reason to be away from you.

A divorce petition may be filed in the family court on grounds of adultery and cruelty. In case you have taken some dowry return it to her with receiving. Also a complaint should be given against her by your mother of domestic violence so that she leaves your house and then you can get rid of her.

Regards 

See firstly her husband and you can collect few evidence of her relationship without being informed to her like video of she fights, text messages with other guy or if she is meeting someone and further a police complaint can be given against her and a divorce on ground of cruelty can be filed with the permission from the court,

See get some more evidence to prove that she is in adulterous relationship with the third guy and then a divorce can be filed against her with permission of court.

It may be time for Marriage Counseling,Marriage counseling can help you rebuild your relationship. Or decide that you'll both be better off if you split up. Either way, marriage counseling can help you understand your relationship better and make well-thought-out decisions.

You can file a application for judicial separation in court and then after a year you can file divorce

Mere sending of messages by a guy to his wife expressing his love for the latter is not a blemish on the girl. Your brother has to dig deeper and find out if she has an extra marital affair and then only he can seek dissolution of marriage on the ground of adultery. 

If their marriage has not been  consummated because she refused to cohabit with him nor she is interested in having sexual relationship with him, he may file a petition seeking to annul the marriage on the same grounds.

Let she file any type of criminal cases agaisnt him he can take support of this annulment case filed against her and can express that she is taking revenge on this basis.

Discuss with a local advocate and proceed on the basis of the recommendations made.

Forget about tracking her phone because it may be considered as an offence from your side for invading her privacy.

If she is not interested in continuing the married life and she is also not allowing him to consummate the marriage, it is better that he may file an annulment petition.

Trying any other option may be a costly affair hence better  take wise decision at right time.