Could toxic chemicals from a brand new plastic hose have
leached into the water that was used on the plants? (Husband says no, wife
says water appeared milky).

I doubt if anything in the hose would have killed the plants.

If repairs to mains water pipes or some other work allows air to enter the
system, when water is restored the pressure causes that air to enter into
solution. When that water - air super solution issues from a tap the air
escapes solution and forms millions of tiny bubbles and the water looks
milky so it most likely had nothing to do with the problem.

An interesting point (completely OT) Charles Dickens would have said "bran
new" or was it just a typo I saw?

Regards

Ron