re: Oak tree issues

Yes it was the only limb in his pics, and it definitely has fungus growing on it, which only grows on dead wood. If a limb that big up in a 50-60' oak tree has that much fungus and rot, and he clearly states there are numerous other limbs falling which are similar size, then he really needs to get an expert opinion on what to do.
I was simply advising him, as I have been in a similar situation with a massive oak tree (70-80' tall with at least 7-8' diameter trunk). It went from looking healthy to shedding large limbs with a lot of recently dead foliage in a matter of months.
Had a long established local tree service come give an opinion. He wanted to set up a treatment plan for ALL my trees, starting at $400 for the first application, then 150-200 for follow ups (as needed).
I was skeptical, so called my county agent. He said bring me a 2-3' section of a fallen limb about the size of your arm and any twigs/foliage you can get that was attached to it. Turns out the quickly dying tree was ancient and stressed beyond it's ability to recover due to root damage. (We had built our house in 2004-2005 and this was in August 2007 during a horrible drought in North AL). Root system was definitely disturbed during construction.
Our 3 other oaks were also shedding limbs with smooth black patches on them and quickly dying foliage. It is hypoxalon canker, which is un-treatable and rears it's head when moisture content in the tree drops below approx 70%. We have had a couple of other droughts since which weren't as sever/prolonged, and lost a few limbs here and there, but no major issues.
My point being, get an expert who has no financial incentive, to give an opinion. Saved me a ton of money.