The key thing to understand about him is he will never attack someone that has not attacked him first.
In this case, she never attacked him. So he offered her twice the amount that had been previously offered. When his partners saw he was unsuccessful, they determined his approach was weak, and they went after her using their lawyer’s argument.
He didn’t like that his partners thought he was weak and ineffectual, and that they had maligned him in the business society (local news reports). When they lost their lawsuit, it is said he received a communication via lawyers that his partners were going to sue him. They attacked him first, so he bankrupted them.
When Donald loses out on a deal, he reacts emotionally and becomes frustrated. If his interest had been attacked, he will fight back with a vengeance. If he had not been attacked, he will walk away pissed. In this case, his partners refused to walk away but lost anyway, twice.
OK, so there is some murkiness around the ownership structure of the Atlantic City properties as he restructured multiple times and lost equity due to the properties failing. However, even as of February 2013, he owned 9.8% of the Trump Plaza, and prior to that the ownership level was much higher:
http://www.latimes.com/business/hiltzik/la-fi-mh-atlantic-citys-implosion-20140714-column.html
As for whether or not he was involved or standing back in this case, it is clear from the contemporary interviews and quotes that he was very involved.
There is the Stossel interview, quoted here:
http://www.thelibertypapers.org/2015/06/17/donald-trump-corporatist-bully-re-post/
One key exchange:
“Donald Trump: Do you want to live in a city where you can’t build roads or highways or have access to hospitals? Condemnation is a necessary evil.
John Stossel: But we’re not talking about a hospital. This is a building a rich guy finds ugly.
Donald Trump: You’re talking about at the tip of this city, lies a little group of terrible, terrible tenements – just terrible stuff, tenement housing.
John Stossel: So what!
Donald Trump: So what?…Atlantic City does a lot less business, and senior citizens get a lot less money and a lot less taxes and a lot less this and that.”
some similar quotes here from Trump:
http://www.nytimes.com/1998/07/21/nyregion/widowed-homeowner-foils-trump-in-atlantic-city.html
It’s hard to conclude he “wasn’t” heavily invested in the effort to take the land through eminent domain