Tenant Invested $5k Based On Lease Signed By Only One Co-owner—can It Be Invalidated? (louisiana)

Location: Louisiana

My mom and uncle inherited this piece of land from their late father. It's not a great piece of land. It's small, it floods frequently and needs a great deal of upkeep. In the past, the parish has had to come and work on it and charged them $1500 to do so. My mother lives out of state and my uncle doesn't have the time to maintain it. For legal reasons, they cannot sell it or collect rent on it.

A while back, one of the neighbors approached them saying that he would like to move onto the land. He promised to clean it up, but wanted a long term lease. My mother and uncle agreed. The neighbor spend upwards of 5 grand (plus lots of elbow grease) to get the land cleaned up. The promise was that he would live on the land, maintain it, and pay the property tax.

Right when it was time to move onto the land, my uncle's wife said that she didn't think it was wise to enter into a long term lease and wanted to do the lease for a year. The neighbor was understandably upset about this as is my mother. They only needed one signature for the lease to be signed. My mother went and signed it. Neighbor is in the process of moving onto it when uncle found out. Uncle is now telling neighbor, he'll be hearing from his lawyer.

Can the lease be invalidated? I assume so. Will the neighbor have any recourse? He invested a good deal of money into this as well as his own time and elbow great.

Not much of this was in writing beyond the lease and some emails (I believe).