Please note that at JMIR reviewers are typically acknowledged at the end of the published paper (in case of acceptance and publication, otherwise they stay anonymous).
So the way how to handle this depends on who wants to be acknowledged:
a) if only you want to be acknowledged and the other reviewer doesn't want to be acknowledged (please confirm with him/her), no action is required. Simply fill in the review form. In the comments for editor, you should disclose who wrote what part of the review. However, this will not be acknowledged in the manuscript
b) if only the other reviewer wants to be acknowledged, please go to the reviewer form and enter the name/email address of the other reviewer under "suggested reviewers", and decline the review request. In addition you may file a ticket with firstname.lastname@example.org requesting the other reviewer to be assigned (state the ms ID and the fact that you entered the other reviewer as suggestion). The editorial assistant will then assign the suggested reviewer
c) if both should be acknowledged, please proceed as above (go to the reviewer form and enter the name/email address of the other reviewer under "suggested reviewers") but do not decline the review request but accept it. Also file a ticket. The editorial assistant will then assign the suggested reviewer. When you submit your review, paste the review into one reviewer form, and in the other review form (the one of your student), select "see comments" as decision, and just enter as "review" a note that you worked with another reviewer on this.
Options a or b are preferred.