Tax savings

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?

"any tips on how to reduce client tax obligation"

Depends on how long of a prison term they would be willing to take a chance on. 

It's a little late to do any tax planning.  At this point it is kinda like asking the embalmer what can we do to save the dead guy.  All you can do is have them write the check and tell them to adjust their withholding for 2022 so there isn't a repeat next year.

IRA contribution?

Max out HSA?

Not much can be done after the fact (as has already been pointed out...)