Coordinates are just nodes with zero size, basically. At any rate, fit works as well with coordinates as nodes, so you can just use the relevant parent and child anchors.

\documentclass[border=10pt]{standalone}
\usepackage[linguistics]{forest}
\usetikzlibrary{shapes.geometric,fit}
\begin{document}
\begin{forest}
[S
[NP$_x$
[he,tier=words]
]
[VP x z y
[V,
[V x y,edge={<-}, tier=words, tikz+={
\node [fit=(.child anchor) (!u.parent anchor), draw, ellipse, inner ysep=1.5pt] {};
}
[baked]
]
]
[NP$_z$
[her,tier=words]
]
[NP$_y$
[a cake,roof,tier=words]
]
]
]
\end{forest}
\end{document}
If you need this often, you can, of course, create a style. I would also straighten out the edge on the right, which looks unhappy to me.
\documentclass[border=10pt]{standalone}
\usepackage[linguistics]{forest}
\usetikzlibrary{shapes.geometric,fit}
\forestset{%
circle edge/.style={
tikz+={
\node [fit=(.child anchor) (!u.parent anchor), draw, ellipse, inner ysep=1.5pt] {};
}
},
}
\begin{document}
\begin{forest}
[S
[NP$_x$
[he,tier=words]
]
[VP x z y
[V,
[V x y,edge={<-}, tier=words, circle edge
[baked]
]
]
[NP$_z$, calign with current
[her,tier=words]
]
[NP$_y$
[a cake,roof,tier=words]
]
]
]
\end{forest}
\end{document}

fitwith thetikzkey or add the code directly. What's the problem exactly? What have you tried? – cfr Feb 23 '17 at 17:14V,withV,tikz={\node[inner sep=1pt,draw=blue,fit=()(!1),ellipse] {};}. But the ellipse is containing node V()an its first child(!1). – Bobyandbob Feb 23 '17 at 18:13