Own property with a friend- Friend wants out HELP

[Ashley Abramson]

My parents own a piece of recreational property (no house, but a bathroom/shower and a cabana) on a lake. They own the property with a friend. 50/50 owners. Unfortunately don't have any more details on the agreement (working on it). Sounds like a lawyer drew up paperwork for them but it is very basic. The friend rarely uses the property and came to my parents wanting them to sell the property with him, while the market is high. He said someone offered him $700k for the whole property. 

Problem is my parents have no interest in selling. So their partner wants $350k (aka half of the $700k he was offered for the whole property). Tax assessment if $492k. I can only find one other vacant lot on the lake for sale it is listed at $449k and has been on the market for 472 days. No real comps on the lake as most anything sold on the lake has a home on it.

I am wondering if the friend/other owner has any legal rights to force my parents into anything? Or at this point can my parents just make him an offer to buy him out? If he doesn't like the offer is he stuck or does he have options? The property has been in our family since the 50s so it is sentimental.