I believe it is good standards to follow the three principles of (knowledge, necessity and benefits), including referral to a research ethics committee that can be a procedural safeguard. The researcher should be prepared to deal with any difficulties of valid consenting that may be the result of a client psychopathology. In research the experimental design should incorporate proposals that cover both general communication (use of appropriate language, repetition, a time for questions and answers). The overall process is ethical as well as heuristic affecting the way the client understands the nature of the research procedure, in the long run it may affect the results achieved.
Fulford, K. W., & Howse, K. (1993). Ethics of research with psychiatric patients: principles, problems and the primary responsibilities of researchers. Journal of Medical Ethics, 19(2), 85–91.
Vaz, M., & Srinivasan, K. (2014). Ethical challenges & dilemmas for medical health professionals doing psychiatric research. The Indian Journal of Medical Research, 139(2), 191–193.