my best friend is attracted to his cousin,wat do tell him?

My friend married her cousin ten years ago and they are still happily married, with their families' blessings. Her cousin is the son of her mother's sister. She married him because she had encountered many relationship problems and her cousin is the only man she can trust. The only restriction she has with this marriage is not to have any offspring. It is known that close relatives have some similar genetic structures which, with fusion, may cause the baby to have down syndrome or other defects. Personally, I'll find it quite weird to fall in love with my cousin because he'll be like a brother to me.