Question Summary:
Can he marry the daughter of his grandmother’s half-sister?

Question Detail:

 My friend is currently in an confused position, and so has approached me to help him, and I thought this would be the best way to find him a solution. He has fallen in 'love' with a girl, of similar age, only one year older than him, and wishes to marry her in the future years. However that women in relation to him is his aunty, but she is his grandmothers, half sister's (so same dad but different mum) daughter. Is it possible for him to marry her or not? 

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
Yes, he can marry her. The girl in question is a cousin of one of his parents, so your friend is not a mahram to her (a mahram is one with whom marriage is permanently unlawful). [i]
Since he is not her mahram it is not allowed for him to communicate with her until after marriage as he is a stranger to her and vice versa.
And Allah Ta’āla Knows Best
Sohail ibn Arif,
Student Darul Iftaa
Chicago, USA
Checked and Approved by,
Mufti Ebrahim Desai.
 
وتحل بنات العمات والأعمام والخالات والأخوال [i]
  رد المحتار، ٣/ ٢٨