This appeal, which concerns the treatment of participant
expert evidence by a trial judge in a civil jury trial, illustrates the
importance of scrutinizing such evidence to ensure that the
participant expert does not exceed his or her proper role.
 Maryvale Adolescent and Family Services (“Maryvale” or
the “appellant”) was found vicariously liable for sexual assaults
alleged to have been committed against the respondent, Jesse
Imeson, by its former employee, Tony “Doe”. Although the
respondent testified that he was also sexually abused by Father
Horwath, a deceased Roman Catholic priest, the jury did not
accept that claim, and the action as against the Roman Catholic
Episcopal Corporation for the Archdiocese of London (the “
archdiocese”) and Father Horwath was dismissed.
 The appellant’s central argument on appeal is that the trial
judge erred in admitting the opinion evidence of Dr. Kerry Smith,
a mental health clinician who treated the respondent and who
gave evidence at trial as a participant expert. As a participant
expert, he was exempt from the requirements that apply to litigation experts under rule 53.03 of the Rules of Civil Procedure,
R.R.O. 1990, Reg. 194.
 The appellant says the trial judge’s error was compounded
by her instruction to the jury to consider such evidence in determining whether the sexual assaults occurred.
 The appellant also takes issue with the trial judge’s decision
to admit into evidence both Dr. Smith’s oral testimony and the
summaries he had prepared based on handwritten notes, which
contained details of his treatment sessions with Mr. Imeson, as
well as various comments and opinions.
 For the reasons that follow, I would allow the appeal and
would direct a new trial of the action concerning the alleged sexual assaults of the respondent by Tony “Doe”.
 As I will explain, Dr. Smith ought not to have been permitted to provide opinion evidence that went to the issues of whether
the alleged sexual assaults occurred (liability) and whether
Mr. Imeson suffered harm that was caused by such assaults (
causation). The expert opinions that were elicited in this trial for
such purposes went beyond Dr. Smith’s role as a participant
expert under Westerhof (Westerhof v. Gee Estate (2015), 124 O.R.
(3d) 721,  O.J. No. 1472, 2015 ONCA 206, leave to appeal to
S.C.C. refused  S.C.C.A. No. 198, 2015 CanLII 69447).
Under Westerhof, a participant expert’s exemption from rule
53.03 is lost to the extent the expert’s opinion is not based on the
expert’s observation of or participation in the relevant events and