Simple answer: if you want to not divorce her but still get rid of the guy, do the following—fast-travel to the college of Winterhold with her as a companion. Next, dismiss her. Before she leaves, speak to her and click out of conversation rapidly. While she is still talking, click the wait-time button and wait at least three hours. Aerin will appear next to her. Click Aerin to speak with him, and then rapidly exit out of the conversation. While he is still talking, press the wait-time button and wait at least three hours. Mjoll is now gone and Aaron is the only one remaining. Now, you could kill him directly, but you could pick up a bounty or Mjoll find out (plus, that would be too easy)—you may walk him to the ledge and push him off by sprinting into him and causing him to stumble off, plummeting to the ice below. Mjoll’s boyfriend is toast (very cold toast of unknown origins in Winterhold, but toast, nonetheless), she knows nothing of what happened to him, you have incurred no bounty, and you’ve been a creatively-homicidal maniac with sound enough justification to be at-least understandable—or, to be more dull and passive about it, you could just divorce her and lose both vengeance and the woman, but this sounds way more fun for a gameplay-option. Maybe it’s just me, but I feel like a gentleman-crook with a penchant for the sly and a dash of deception with this option, and it makes for quite a playthrough. My characters are always such wonderful people…they really, truly are….