Your strings sound right, though the 9" brace on the 52" bow shows that the rule of thumb doesn't hold for every bow. My K-mags take a 48" string for starters, but they brace way lower than 9"!
My course of action would be to try the bow with the 8" brace, and see how it does, seeing as you have the string. 9" sounds too high to me.
Trouble with endless strings is that they don't lend themselves to tweaking. Flemish strings for your bows would allow you to twist or untwist to see what the trend was on raising or lowering the brace. Then, if you prefer them, make the correct length endless type.
Or, how about a single "test" string, with one loop, and fastened with a bowyer's knot on the lower limb, to try the length? One long Flemish twist string like this could be used to dial in the correct brace on any bow, and you would then make the proper-length string to replace it.
1816 or 1916 arrows cut to 28" should be fine for your buddy, with 125 grain heads. I draw a little longer, and use up to a 2013 with bows in that range.
Killdeer