How to worsen somebody's motivation?

The absolute best way to worsen, even annihilate the motivation for a person is to convince him or her that all his/her efforts are worthless. Once that person believes that all the actions made by him do not make any effect on anything whatsoever, he will stop doing anything. This is just natural response from human mind: "Why do something useless? There is no point of willing to do that."

Now, how to convince someone of that depends on the context: the subject being demotivated, his area of interest and the one who contacts with the subject to demotivate him. I think a decent example of a motivated person is an annoying child who tries to gather somebody's attention by irritating them. It can be surprising how much energy kids have when it comes to making people focus on them, which from my perspective qualifies as a motivation property. It's a well-known strategy to ignore such children if you are their subject of interest. Once they realise that no matter how hard they try to attract your attention, you do not care about them, they find something else to do and likely leave you alone. That happens as a result of them getting demotivated.

Another way to demotivate someone, perhaps less effective though, is to deprive him of the reward he hoped for for achieving something. This works for those whose motivation is bolstered on something tangible that comes from the outside. Think promotion for hard work, for example. If you want to have an employee who is not motivated, let him know that he gets little to nothing for his efforts. Note that those efforts do not have to be meaningless on their own, like in the first demotivation idea. Your fiance probably does realise that a gift to your mother was good for her, so it's totally not worthless. But if she did that mainly to impress you, then unless you are willing to show your appreciation for that, chances are that your partner won't be motivated to invest into the good relationships with your parents again.