What is the best way to get my wife’s putter to the right length?
Dave, your question can be taken in more than one way. It may be a putter fitting question, “how can I tell what length putter is right for her?” Or it can be a club repair question, “how can I shorten or lengthen her putter?”
So here’s a link about putter fitting: http://ralphmaltby.com/how-to-properly-fit-a-golfer-for-putters/
Of course, changing the length of a particular putter means either cutting the shaft to the desired length and then regripping, or adding a shaft extension, then cutting to length and regripping. Here’s the link for one of the available shaft extensions: https://www.golfworks.com/steel-shaft-butt-extensions/p/S60/
Hope this helps! Michael