Parents had a bad leak coming in from the basement. Took off some the deck boards and found the siding tar paper was completely shot. Replaced the paper with GAF tiger paw (its what I know) and that fixed it.
Found out there was never a transition piece between the sliding door and the deck. Sun damage probably is what caused this failure. Found a piece of scrap wood, cut it to rough size and then hand plained it to fit.
Ended up finding a second spot that was causing a soft spot above the floor below we didn’t know about. Fixed that too.
Saved all but one of the boards. Most of the screws were stripped.