While I disagree with the "harshness" used, I somewhat agree that this doesn't seem quite legit either.
The applicant accepted PR status and came to Canada in 2012. In order to have not been lying, he/she would have needed to be still in a genuine, on-going relationship with the sponsor at the time of landing. If he/she was NOT, then this is fraud and PR would not have been granted.
Assuming the applicant and sponsor were still in a proper, genuine relationship when the applicant arrived as a PR, in a short time the relationship completely deteriorated, someone cheated, the OP filed for divorce, healed the wounds of a marriage he/she worked 2+ years on getting PR for, found a NEW love interest, grew their relationship to the point of knowing 100% that he/she wanted to sponsor this person and commit to being responsible for them for 3+ years, and has started to look into the details of applying for this new love interest before his/her divorce certificate for her PR-gaining marriage has even arrived.
To me... it sounds more than a little strange. While I understand that everyone is different and marriages and relationships (sadly) end in horrible circumstances sometimes, this sounds a little pre-orchestrated to me and a little strange for so much to happen in a short period of time.
Also, the OP stated that the "sponsor" wanted divorce because it took the applicant 2 whole years to get to Canada and the sponsor had an affair... If the sponsor chose to divorce because of the 2 whole years, that would imply that the relationship was already on the rocks when applicant landed (since sponsor wasn't willing to wait the 2 years and wanted to divorce for that reason).
All just speculation, of course, and I'm not sure that the details really matter since a person's personal life isn't really my business. But it IS stuff like this that makes some of the more typical cases and obviously legit families be kept apart for years and have their applications take forever to process. Sigh.