Unfortunately I am an Authority on this subject. In fact, I may be starting a blog/website devoted to relationships/marriage/adultery.
Your brother did not cause this affair. His girlfriend is 100% responsible for HER affair. The same as if she robbed a bank and your brother did not know about it.
The majority of people who have affairs have just suffered some sort of trauma. Job loss, death in the family, bankruptcy, medical issues, etc... they have very Low self-esteem.
Happy people don't have affairs.
Most affairs (90%) happen in the workplace. It's the people they spend the most time with. Affairs start off as an Emotional connection, then it leads to the bed.
Most people don't start an affair intentionally, it "just happens" without them realizing what's going on.
The easiest way to catch a cheater is to check their cell phone records. 95% get caught this way. Also look for prepaid phones (not always used for Paypal).
Here is the True Test as to someone truly wants to repair the relationship. If the cheater does not comply then run to the nearest door. Hit the road jack.
3 things a Cheater must do to reconcile their affair.
1. A true and sincere
Apology (tears are almost mandatory, no crocodile tears).
2. Sincere
Remorse. A person needs to do everything required over a period of time (6-12 months) to show they are truly committed. Allow the checking of cell phone records, passwords to al email accounts, facebook, etc...
Always keep the betrayed partner (your brother) aware of their location, trips, overnight stays out of town, etc... they must answer their phone when called.
3.
Answer Any & All your brothers questions regardless of how disturbing and private they may be.
Most women want to know if their spouse/boyfriend truly loved the OW (Other Woman).
Most men want to know about the details of their sex life. The majority of men want to know all the disgusting details.
His girlfriend is not allowed to "think" about her answers. Questions must be answered within 1/100th of a second. If she needs to "think" then she is Lying.
The answers will hurt your brother. However, he has a Right to decide if he wants to pursue the relationship. It's not just the Adulter who has rights.
Most cheaters think the spouse doesn't need to know. However, the spouse has a right to decide if "they" want to stay in the relationship once ALL the FACTS are on the table.
Print this out and give it to your brother. If he does not get his girlfriend to comply with all three then walk, today. It will happen again if not addressed.
Affais are Addictions. Once people get involved in an affair it's very, very hard to break the cycle. Very similar to a drug addict. Emotional affairs are the worst, especially for women. If your wife/girlfriend get's Emotionally involved with another male then your in for the Biggest Emotional Roller Coaster ride of your life.
Relationships can actually get stronger post affair. But it's not something anyone ever wants to go through.
I wouldn't wish is on my worst enemy.
If your gut tells you your partner may be cheating, then your probably right. In many cases the Signs were there long before we found out. We simply overlook them because we are in denial or don't want to ruin what we "think" we have.
I wish your brother the best.
.
It's a long and tiring process to go through to heal the relationship. Don't simply walk away in most cases as some would suggest.
Their is no guarantee your next partner won't do the same thing.
Family relationship dilemmas 
