Audit Reconsideration

I have a new client (friend of my family) who filed her own return, the IRS questioned her EIC, she sent the IRS some of the documents they requested but not all, and the IRS disallowed the EIC & HOH and she just received a notice of deficiency. This is an old lady who raised all of her grandchildren because her daughter (the kid's mother) is a drug addict & abandoned the kids many years ago. She came to me with the proof needed (school records, medical records, utility bills, homestead exemption, letter from the school, birth certificates....). So my question is, what is the best route to go with that information, not even sure the IRS received any of it, she sent a few of the items to IRS with no response as to whether they received it or not, but she didn't send all of the documents I now have. Is Audit Reconsideration the quickest, simplest route to go on this?

the IRS disallowed the EIC & HOH and she just received a notice of deficiency.

You can send the information to the office that issued the notice of deficiency, but if you don't get a response and the deadline for filing the Tax Court petition gets close, then file the petition. You can download a set of forms and instructions from the Tax Court's website here:
https://www.ustaxcourt.gov/forms/Petition_Kit.pdf

Audit reconsideration applies AFTER the tax has been assessed and hasn't yet been paid.

Unless you're a member of the Bar of the Tax Court, the client would have to sign the petition. As you may know, however, once the Tax Court petition has been filed, the case would be assigned to an IRS Appeals Officer, and you could represent the taxpayer at that preliminary stage (under a Form 2848) without being a member of the Tax Court Bar. Chances could be reasonably good that the matter would get resolved at Appeals, so the case would never go to trial.

EDIT: And, as dave said, don't let the client get too close to the 90 day deadline before filing a Tax Court petition. A couple of years ago, I had a friend come to me, out of the blue, on the evening of "day 88" with a Notice of Deficiency. Wayyyyyy to close for comfort! I managed to get his petition mailed (certified mail, return receipt requested) on literally the last day. (It ended well, too. We turned an asserted deficiency of over $35,000 into a net tax refund of over $2,000.)