Quit being such a nosy person. If your friend wanted to know his wife's background, his wife would have told him before they got married during the latter stages of their courtship, or he would have found out the information himself through background checks prior to the marriage.
If you were truly a friend, you'd have learned to MYOB when it comes to the personal lives of others, especially those whom you profess to care for. Do you have some sort of secret desire to breakup a marriage where both husband and wife are otherwise comfortable with the state of things the way they are? Perhaps you have jealousy issues, where you have decided that if the past relationship between you two was diverted by the presence of his wife, then you would do your best to breakup the marriage and punish your friend for making you less important in his life. Your friend's wife is his SIGNIFICANT OTHER. It's just too bad that you aren't as significant in your friend's life as you once were. I certainly hope you aren't motivated by malice or revenge against your 'friend' and his family. Anyways, Transphobia is not the basis for long lasting friendships. I seriously doubt if you'll remain a friend of the husband if you continue with your actions.
By the way, the person your friend married is not a "Transgender". She is a WOMAN. Her status with your friend in their marriage is that of Wife, or Spouse, or even Significant Other. How would you like it if others started to refer to you by the designation of NOSY BUSYBODY?. I guess if the definition fits, you should wear a label too.
A cisgendered heterosexual male who wonders, if I had 'friends' as inconsiderate as this, would I really need to search to identify my enemies?