\renewbibmacro*{in:}{%
\setunit{\addcomma\space}%
\ifentrytype{article}
{}
{\printtext{%
\bibstring{in}\intitlepunct}}}
\renewcommand*{\intitlepunct}{\addspace}
is the correct solution for what you want.
But there is a slight catch to this that becomes apparent when we use the code with the minimal example from your question Why can't my MWE see the references?.
If you have additionally modified the field format of the title field (I'm speculating here, because you have not shared an MWE, but it is not entirely outlandish that such a thing happened because you must have modified the style heavily to obtain the output you show) the periods that you placed at the ends of the fields of froestad2015Criminology could confuse biblatex's punctuation tracker.
\RequirePackage{filecontents}
\begin{filecontents}{\jobname.bib}
@article{friedman2001greedy,
title = {Greedy function approximation: a gradient boosting machine},
author = {Friedman, Jerome H.},
journal = {Annals of Statistics},
pages = {1189--1232},
year = {2001},
publisher = {JSTOR},
}
@incollection {froestad2015Criminology,
author = {Froestad, J. and Shearing, C. and {Van der Merwe}, M.},
title = {Criminology: Re-Imagining Security and Risk.},
booktitle = {Security: Dialogue across Disciplines.},
publisher = {Cambridge: Cambridge University Press.},
year = {2015},
pages = {177--195},
editor = {Bourbeau, P.},
}
\end{filecontents}
\documentclass{article}
\usepackage[%
backend=biber,
style=authortitle,
sorting=nyt,
]{biblatex}
\addbibresource{\jobname.bib}
\DeclareFieldFormat[incollection]{title}{\enquote{#1}}
\renewbibmacro*{in:}{%
\setunit{\addcomma\space}%
\ifentrytype{article}
{}
{\printtext{%
\bibstring{in}\intitlepunct}}}
\renewcommand*{\intitlepunct}{\addspace}
\begin{document}
\textcite{friedman2001greedy,froestad2015Criminology}
\printbibliography
\end{document}
gives

biblatex tries to suppress double punctuation and so the full stop at the end of
“Criminology: Re-Imagining Security and Risk.”
suppresses the comma that would normally follow it because a full stop can not be followed by a comma.
The effect is not really visible (if you look really closely it is actually) for the
Security: Dialogue across Disciplines.
because that would be followed by a full stop anyway (but if you have the . in the booktitle field the period is in italics, while it is upright if biblatex adds it itself).
The comma is not suppressed in
Cambridge University Press., 2015
because biblatex (incorrectly in this field) assumes that a . in the publisher field always signifies an abbreviation after which a comma is fine.
The solution is to remove the superfluous periods from your .bib file.
@incollection {froestad2015Criminology,
author = {Froestad, J. and Shearing, C. and {Van der Merwe}, M.},
title = {Criminology: Re-Imagining Security and Risk},
booktitle = {Security: Dialogue across Disciplines},
publisher = {Cambridge University Press},
location = {Cambridge},
year = {2015},
pages = {177--195},
editor = {Bourbeau, P.},
}
works just fine and gives the expected

.bibentries. – moewe Jul 03 '18 at 15:50titlefield of the@incollectionreadstitle = {Criminology: Re-Imagining Security and Risk.},with a full stop at the end. That confusesbiblatex's punctuation tracker. Remove it and the comma should appear as desired. – moewe Jul 03 '18 at 15:54biblatexindentation style, it is a bit idiosyncratic at times. – moewe Jul 03 '18 at 16:25\setunit{\addcomma\space}%go? You should be using the third code block of my answer (mangled version:\renewbibmacro*{in:}{% \setunit{\addcomma\space}% \ifentrytype{article} {} {\printtext{% \bibstring{in}\intitlepunct}}}) That definitely works as can be seen from https://gist.github.com/moewew/02246a34f3dbeef82c7d908c982bf81c – moewe Jul 03 '18 at 16:32titlefield of your@incollection. Remove the.so that it only readstitle = "Criminology: Re-Imagining Security and Risk",and then try again with the code from this question. – moewe Jul 03 '18 at 16:37\renewcommand*{\intitlepunct}{\addspace}– moewe Jul 03 '18 at 17:01