Nulle Construction

It was for my parents. He did a good job on the construction of their addition, but a window was broke. As the window has lifetime warranty, he ordered another one and claims he has it (5 years or more now). However, each time my dad has called he has come up with excuses as to why he cannot come out to install it. He still has the original paper for the window or my parents would use that and order it themselves or go through someone else. I would like to take care of this for them as someone need to get this guy moving on replacing that window that doesn't work and is under warranty. If he has, why would he hold onto it. If he doesn't, give them the paperwork to find someone else to put it in. They mention it to us kids about every other time we visit and it get boring listening to them gripe about it. They haven't contacted them recently as I think they are under the impression, they will get the same lame excuses, but if I were to contact him, I would just request the paperwork and be done with him.