First off, I assume the marriage between your aunt and the man did not produce children.
The moment she married this younger man, he gained the right to at least a significant portion of your deceased aunt's share.
I assume she had children from a prior marriage. If not, he is entitled to half of your aunt's share, meaning 25% of the entire house. If she did have children, he would receive one-fourth of her 50%.
If you want the house, buying him out seems like the most civilized option. In the end, he does have a right to at least a small portion of the inheritance. It may seem unfair, but under Islamic law, a husband of one day and a husband of 50 years have the same rights.
This is probably the first time I've heard of a Somali woman marrying a younger man, its usually the other way around.