"What if this is the first nondeductible IRA?"

That's not enough info. You stated this: "I have a client that contributed in 2021 $7K to a traditional IRA" and that there is SEP. Was there any other money in any Traditional IRA, including any earnings? The SEP? Was there Basis in any of these accounts? Because if Not, if a conversion (which is what a backdoor Roth functionally is accomplishing) includes account(s) with anything other than Basis, the conversion will be pro-rata.

You also stated: "and then converted in same year"

That's also not good enough to help. Was it right away or later? Later, as in, now there are sheltered earnings that create a pro rata conversion.

Remember this: "A "backdoor" Roth simply represents an amount put into an IRA post-tax, and immediately converted, avoiding any earnings that would be taxable."

"So line 1 on Form 8606 would be in this case $7,000

Line 2: Enter your total basis in traditional IRAs." <== Basis, because it is post-tax

"All of her IRAs were deductible in the past."

And here is your problem, then. Not Basis. Contributions not basis; earnings not basis.

"So this is pretax money." <== meaning Not Basis

"Only the 2021 IRA contribution of $7,000 is nondeductible so this is post tax." <== Basis

It is Pro rata (lines 10 and 11). And make sure to understand how Line 6 includes SEP, because you mentioned this person has SEP and not only IRA. I mentioned for some things, they are combined/included.

"Do I need to ask the client for the basis in her traditional IRAs(pretax accounts) for this line?"

Of course. But you already stated 0 prior to 2021. If it was All Basis, you would be fine. If there is any other Basis, add it to the $7.000 figure.

Perhaps we should review that Basis is the total of any "already taxed" monies. Any amount sitting in any IRA/SEP/SIMPLE for this person, for amounts never taxed (which would be pre-tax contributions and all earnings) will result in a pro rata condition for the Basis amount you want to show converted to Backdoor, because it is not selective. You can decide the Amount is $7,000, but not which $7,000 that will be. It's just a number.

Example:

I have put $20,000 pre-tax into IRA over the years, and I have $1,000 (sheltered) earnings. I now put $7,000 post-tax (to be Roth backdoor conversion) into a new IRA (which you don't have to do for Backdoor, but perhaps I want to work with a new broker for the Roth). Before I convert $7,000 that is the entire new account, then, I have to look at the big picture:

$20,000 + $1,000 + $7,000 = $28,000 in all of my IRA accounts

Now, let's convert $7,000 (which is computed as if it will be from any/each/all of my IRA accounts). That's the point; the IRS treats all of the IRA accounts as if they are one.

$7,000 out of $28,000 is Basis, or 25% of every $1 converted was already taxed (is Basis). The other 75% is taxable as income.

$7,000 (amount to convert) X .75 = $5,250 is taxable; $1,750 was from basis and reduces my Tracking of Basis. $7,000 (was my Basis) minus $1,750 basis converted = $5,250 of original basis still left in IRA.

Next year, if there will be conversion, the math is:

$5,250 divided by whatever is my new Total. If I decide to do another $7,000 backdoor Roth, then the Basis is $12,250 against the Total of my account(s) for the factor to use for the amount I am going to convert, which (by the way) does not have to be $7,000.

Remember that contribution limits are not applied to Conversions. She can put in $7,000 and the convert All of the IRA amounts, as long as she is prepared to pay the taxes on that larger amount. So, perhaps that is another confusing point for you? Contribution Limit vs Conversion Amount.

Whew. I think that covers all of this... I'll edit it if I spot a mistake. Good Luck.