One important thing is missing in this hypothetical question: WHO initiated the borrowing. If the owner unilaterally offered ("hey, use my rod"), or if the other person asked ("pal, can I borrow your rod"). If the owner offered the use, he assumes the loss. If the other person asked to borrow it, then that person assumes the loss. This is a technical and somewhat legalistic analysis - but the way it would work looking at it that way.
The more practical approach in my view would be this: if a guy lends his rod to a friend no matter who asked or initiated the lending, the owner just takes the loss as a necessary part of friendship and life.
On a final note, I would agree that the 15 pound rating would have utterly no effect on breaking a rod. Frankly I cannt imagine a rod actually breaking if the user is playing a fish properly, no mater the line poundage. In my 60 years of fishing (meaning I started at age 6 and am now 66) I have never seen a rod break while fighting a fish - seen the tip eye break a couple times because it was worn or damaged, but never even the cheapest rod break.
Anyhu, that's my two centavos. Stay safe and hanapa'a. tom m