You should speak with a Real Estate attorney in your area to consider if there are laws specifically in place regarding this question.  Generally speaking, each co -  tenant in a tenant -in-common scenario has equal rights to occupy and improve property.  If they do so without the consent and agreement of the other co tenants then then can not seek contribution for what they have done.  But the other co tenants can not reap the benefits from same either. Sounds like he has put in a lot of time and money to improve the property.  Good luck.