An American trust fund heir has adopted his lover in a bid to save his fortune from a couple who are suing him over the death of their son.

The International Polo Club Beach founder, John Goodman, is accused of drink driving in an accident that killed 23-year-old Scott Patrick Wilson, Fox News reports.

In February, 2010, Goodman allegedly ran a stop sign and shunted Wilson's car into a Florida river, and his parents are suing Goodman for wrongful death.

A judge previously ruled that money left for Goodman's two biological children would not be included in part of his personal wealth, the Palm Beach Post reports.

Since 48-year-old John Goodman adopted his 42-year-old partner Heather Laruso Hutchins late last year, Hutchins is entitled to a third of his trust.

The Wilson family's lawyer says Goodman is trying to use the adoption to shield his assets.

"It cannot go unrecognised that (Goodman) chose to adopt his adult girlfriend as opposed to a needy child," he told the Palm Beach Post.

Adoption lawyer, Jeanne T. Tate, told MailOnline that there are many cases where adoption "is used for purposes other than what you'd traditionally think it was intended."

"There's nothing illegal, per se, about adopting your girlfriend," Ms Tate said.

Circuit Court Judge Glenn Kelley wrote that the case takes "the Court into a legal twilight zone".