Hello,
Thanks for posting your query at DoctorSpring.com
Forgetting one pill does not make her particularly at risk for pregnancy.
She would still be protected to a reasonable extent.
There can be no percentages quoted here, as there are no studies concerning the same.
However, from what I make out, she forgot a pill on day 18, and then you had unprotected sex about a week later, making it atleast day 25.
She would definitely not be ovulating then, so your risk of pregnancy is quite negligible.

All the best
Please feel free to discuss further.

Hello again.
Let me clarify further upon your request.
1. Owing to the cycle day she was on, yes, she would be reasonably protected even if she failed to double up the forgotten pill the next day.
2. The one missed pill would not affect the placebo days, since the range for ovulation is past.
3. Now regards your query.
Generally in a 28 - 30 day cycle, ovulation happens right in the middle.
Around day 14.
The range can be extended by 2 - 3 days, so say, maximum day 17 - day 18.
Now , what birth control does, is inhibit ovulation totally.
So inspite of one missed pill, that effect would stay, that is, ovulation would not happen.
NOw we assume that due to the one missed pill, the birth control went awry and ovulation happened, which might lead to a pregnancy .
Now, that would have happened only before day 18 of the cycle, which is when she missed the pill.
You had sex about a week later, when it is just impossible for her to ovulate.
So you see, technically, it is not likely for her to get pregnant from the missed pill.
Hope this explanation helped.
Let me know if you wish me to specify further.
Take care. !