About the saver's credit: it has some AGI "cliffs" where reducing your income by a single dollar could cause the credit to go up by a few hundred. The first such cliff is from a $0 credit to a $200 credit. This happens at $30,750 AGI.
For a self-employed person with $40,000 of business income, they will pay $5,652 in self-employment taxes. Half of this is deductible from AGI, leaving your friend with an AGI of $37,174 before other deductions. The regular tax at this income level is 15%, so a traditional retirement contribution of $37,174 - $30,750 = $6,424 would result in a tax savings of [$200 + (15% * $6,424)] = $1,163.60. That's an 18% tax savings on that money, which isn't bad.
Putting this money in a Roth account instead would then mean your friend would have to pay that $1,163.60 of taxes this year, but it would then grow tax-free. If she expects to have a tax bracket over 18% in retirement, this is a good plan.
The next saver's credit cliff is at $20,000 AGI, bringing the credit up to $400. There's a pretty narrow band until the next cliff at $18,500. Once you get there, the nominal credit is $1,000, but for most people it will just max out at whatever their tax was before the credit. As getting to this level would require your friend to save half her income, it probably isn't on the table right now, but it's good to be aware of if she manages to increase her frugality over time.