So basically it was a case of the other guitarist being too heavy handed on his strumming pattern.
Essentially, yes.
What gauge string was he using? If he wasn't he should have been using 13s as they may have helped his tuning problems.
I do believe he was in fact using a heavier gauge string set that I was. I've always used EBSS .009 on my electrics. I want to say he was using .010s at least. I use 011 on my acoustics.
I was in a band with a different guitarist who was using those .013s. Claimed he liked them for the tone. He played a Les Paul Gold top so not only was he killing his shoulder every night..... yep, he complained about how heavy that axe was..... but he also had strings that essentially prevented any kind of serious string bending and yep... he complained about that as well.