your friend need to understand two things
Firstly, if he strongly thinks that a girl coming into their house can weaken the relation which his family enjoys for more than 3-4 decades then he is wrong. When a new member comes in house everyone has to make adjustments be it a new born kid or a newly wed couple.
Secondly, marriage is not compulsion so if he feels he does not want to marry he should nor marry but if he thinks otherwise you need to make him understand that he is looking for a girl with whose he can openly talk and share his feelings color & looks dont matter at the end, if you two are happy. Hope this helps.