Your friend would have to pay higher taxes as well. Currently she presumably files as Head of Household and her friend files as Single.
In 2017, the IRS says:
Head of Household: $9350
Single or Married filing separately: $6,350
Married filing jointly: $12,700
Personal exemption: $4050
$9350 + $6350 = $15,700
So currently they can deduct the first $15,700 of their combined income. If they marry, that will drop to $12,700. That's a loss of $3000.
Tax brackets, 2017.
Let's say your friend has an income of $50,000 and her friend also has an income of $50,000. So
$50,000 - $9350 - 2 * $4050 = $32,550
($32,550 - $13,350) * .15 + $1335 = $4215
Your friend, with one child, pays tax of $4215 now, in the 15% tax bracket.
$50,000 - $6350 - $4050 = $39,600
($39,600 - $37,950) * .25 + $5,226.25 = $5638.75
Her friend, with no dependents, pays $5638.75 now, in the 25% tax bracket.
$4215 + $5638.75 = $9853.75
So together, they pay $9853.75.
$100,000 - $12,700 - 3 * $4050 = $75,150
($75,150 - $18,650) * .15 + $1865 = $10,340
Post-marriage, they pay $10,340 if married filing jointly.
$50,000 - $6350 - 2 * $4050 = $35,550
($35,550 - $9325) * .15 + $932.50 = $4866.25
So your friend's taxes would go up to $4866.25 if married filing separately. Her friend's taxes would still be $5638.75.
$10,340 - $9853.75 = $486.25
$4866.25 - $4215 = $651.25
That's $486.25 more if they file taxes the cheaper way. Part of this is the $3000 loss in deductions. That's $450. The other part is that the married filing jointly brackets kick in lower than head of household. The borders between 10% and 15% had been $13,350 and $9325 for a total of $22,675 at the 10% rate. But married it kicks in at $18,650. That's down $4025 for another $201.25 in taxes. On the bright side, there was enough left in the 15% that married they saved $1650 in the 15% rate that used to be in the 25% rate. That's $165, less than they lost moving from 10% to 15%. And just a little more income would have put them over the $75,900 and into the 25% bracket.
Turbo tax says that she would lose the child tax credit if married filing separately. I didn't include that, as it's the same for head of household or married filing jointly which is what I compared. That's a big difference though, and tax reform may make it bigger.
The exact amount of the tax impact depends on their exact tax situation. Maybe they itemize and the standard deduction does not matter. Maybe their incomes complement better, so the bracket creep does not matter as much. Or maybe it's worse. I would encourage them to do the math themselves. They already have the 2016 information, so they could redo their actual returns and see what would have happened.
Note: there may be some circumstance under which they actually pay less taxes. However, my reading suggests that such scenarios will be rare. Perhaps if one of them does not work and therefore has no income, but you talk about jobs for both. All that said, the best proof would be to calculate the taxes based on real information rather than an example.