Just to review:

Withholding isn't Taxes or Liability. It's a Prepayment.

If you make a lot, you owe a lot. If you didn't have enough withheld, you might have a balance due with the tax return.

To reduce a client's obligation, that is called Tax Planning and means you want to give financial guidance. That includes that the person should take advantage of resources available, such as retirement plans. You mentioned one dependent; a child might benefit from the client opening a college savings plan account (529).

This is a public internet forum, so you need to be very careful about putting details on the internet for help with this person. Perhaps they (and you) should consult with a planning professional in person?