The minimum requirements for an airplane add-on will be lower if you already have a helicopter pilot certificate, but there is no guarantee that you will finish in the minimum time. You will not require a written test, and you will also need less ground instruction because you will already have some knowledge of things like weather, regulations, navigation, etc. At the end of the day after you have already spent so much time and effort on your helicopter training, the savings on the fixed-wing add-on versus a pilot with no experience will be small, but it will take less time and expense for the average person.
Also, do not do airplanes first thinking you will save money on your helicopter training. That is a common misconception that doesn't work out if you actually tally the costs. If you want to fly helicopters as your career, then start with helicopters.
Caretaker - Sorry for the thumbs down, but I disagree with some of your statements. You make it sound like a helicopter pilot certificate has fewer privileges than an airplane certificate. In fact they are the same except that they apply to different aircraft. For flying airplanes, you need an airplane certificate. For flying helicopters, you need a helicopter certificate. For flying both, you need both. That's it. Sure, more people fly airplanes, but he has already stated that he wants to fly helicopters.
Also, I am not sure what you mean by "more learning aids dedicated to fixed wing". There are textbooks and supplies for helicopters just as there are for airplanes, and there are many that apply equally to both.
I do agree that you should not attempt to train in both at the same time. That will just confuse you and cause problems for both.