I think it would be better just to stay on good terms and let him know you will contact him when you are ready to sell.

Simply put, we start off with the As Repaired/Completed Value (ARV), then subtract from that number a reasonable profit, the rehab cost (scope of work), which we've gotten good at, a contingency reserve for any "unexpecteds", our cost of capital/carrying costs (interest and costs of the leverage used), and our costs/fees on the buy and sell sides of a flip.

Are you planning to sell your motel?

I am considering working with a property developer in NJ to buy land, build a duplex/triplex and sell it (6-9 month time horizon).  

Purchase price: $2,000 Cash invested: $60,000 Sale price: $91,000 Bought this one from WCLB with the intention to fix it and sell on market to homeowner.

Better play it safe and use a warranty deed instead.Also, later on when you sell your property, it will keep the chain of title cleaner for the next title insurance.Last but not least, a warranty deed does not cost more than a quit claim deed...

How could I force her to sell.

Again, the BiggerPockets store has some books on this topic, or you can learn about it by watching podcasts, reading blogs, and interacting on the forum.

If you sell it, you get $250k single / $500k married exemptions from capital gains.Speak with a tax professional about how much you'll save in taxes by doing this. 

With one duplex you should be focused on selling 99.9% of the time and .1% on the duplex.