Those feed systems don't work as reliably as back fed, and the more things going on, the more there is to break, and the design definitely is stressed by heavy papers since they need to be curved at a very steep angle. Some may be fine, but like you said, it is probably that his just isn't able to handle the load of heavier papers anymore, while yours is in better condition and of a higher quality from the get-go, and can.
They aren't ALL are problematic, but overall the design is not as conducive to happy 110lbs printing as other designs.
The 1350 is about ten years old, the c410 is like two or three years old, so age is probably a big factor in the performance difference also, as well as possible improvements in hp's front feeder technology.
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Regardless, I'm glad you found a solution, recon. You can also always save the pep file as a pdf and have her print it, saving you ink and paper, but whichever works best!