This is not a guy who likes to keep a wife around when wrinkles start becoming more abundant.

He might consider her plagiarism of Michelle Obama to be the moment when the moderate republicans (the few who hadn't already realized this) realized that the campaign is in such disarray and run with such incompetence that it wasn't worthy of their support.

He also knows the alt-right probably wasn't too pleased with his wife putting Michelle on a high enough pedestal to plagiarize her verbatim in essentially the same context (speech by wife at party convention). I don't have to tell you what the alt-right thinks of the Obamas. This and the previous point both relate to him losing support because of this speech. Bullies like to blame their failures on people they can easily access and push around. I only hope he isn't already physically abusing her.

He's probably not as rich as he says but he's got enough cash to import a top tier mail order bride.

They have a prenup.

His tax returns would probably reveal assets he has intentionally hidden from her.

Putin could just straight up tell him to divorce her and he would comply immediately.

She might discover his NAMBLA donations and GTFO to protect their son. This would lead to him divorcing her also.