I'm not sure about Substance painter licensing and pricing, but I have used Blender to fix some textures for G8.1, it was few months ago, so I don't recall exact details, but AFAIR I exported the figure to Blender with materials included, opened it up there, switched to Texture painting and fixed whatever I needed there (afair I blended some poorly done seams on a figure), saved the particular texture as a new jpeg and then edited the surface in Daz to use the new texture
I know Substance painter has a lot more to offer, but maybe a free tool works for him
Blender is still free, and featurewise is growing at a very rapid rate, and since the 2.7ish days, the UI has been improved a lot also, I'm a "Sunday" Blender user still, but few tutorials I see on YT make me want to spend more time in it, if only there were 5-10 extra hours in a day...