Is it allowed for a wife to have short hair if her husband wants her to?

So yes, you can have shorter hair or longer hair. The key aspect for females is that the haircut that you have, it should not resemble something which is specifically known to be a haircut for males. So as long as this is not the case, then yes, you can have shorter hair or longer hair. And also, if you do it for your husband, it's also considered an act of ibadah, an act of worship and a good deed for you, inshallah.