I don't really see anything to worry about. It looks like the yellowing is just the older growth dying off. The lighting isn't great in the pic, but it looks like the plant is healthy and the new growth seems OK. Those brown spots on the leaves may be a sign of a slight calcium deficiency, and I find that plants use more calcium during their first few weeks of flower, BUT, if I had to guess, those brown spots look more like you got some water/nute drops on those leaves and they got a little burn in those spots from the lights. The drops act like a magnifying glass and can cause burn on the leaves.
The run off water doesn't tell you the pH of the medium.
How long until harvest?
I see buds, can you post a close up on them please.
If he's PH'ing the solution first, assuming to about 6.5, and the run-off is 6.5, the medium is fine. While run-off doesn't tell you the PH of the medium, by measuring the starting PH of the solution, then measuring the run-off, it tells you what the medium is doing to the solution. If run-off PH was higher, the soil would have to have a higher PH and the opposite would be true if the run-off PH was lower.