littleadv's first comment - check the note - is really the answer.
But your issue is twofold -
Every mortgage I've had (over 10 in my lifetime) allows early principal payments. The extra principal can only be applied at the same time as the regular payment. Think of it this way - only at that moment is there no interest owed. If a week later you try to pay toward only principal, the system will not handle it. Pretty simple - extra principal with the payment due. In fact, any mortgage I've had that offered a monthly bill or coupon book will have that very line "extra principal." By coincidence, I just did this for a mortgage on my rental. I make these payments through my bank's billpay service. I noted the extra principal in the 'notes' section of the virtual check. But again, the note will explicitly state if there's an issue with prepayments of principal.
The larger issue is that your friend wishes to treat the mortgage like a bi-weekly. The bank expects the full amount as a payment and likely, has no obligation to accept anything less than the full amount. Given my first comment above here is the plan for your friend to do 99% of what she wishes:
- Put aside 1/2 the payment when each paycheck comes.
- Make the regular payment if that's what's in the saved pile of cash each month.
- Twice per year, she will see an extra half payment, i.e. 1-1/2 times the needed money set aside.
- When (3) occurs, make the payment with additional principal equal to the 1/2 payment. This will happen twice a year, effectively making 13 full payments a year.
Tell her, there's nothing magic about bi-weekly, it's a budget-clever way to send the money, but over a year, it's simply paying 108% of the normal payment. If she wants to burn the mortgage faster, tell her to add what she wishes every month, even $10, it all adds up.
Final note - There are two schools of thought to either extreme, (a) pay the mortgage off as fast as you can, no debt is the goal and (b) the mortgage is the lowest rate you'll ever have on borrowed money, pay it as slow as you can, and invest any extra money. I accept and respect both views. For your friend, and first group, I'm compelled to add - Be sure to deposit to your retirement account's matched funds to gain the entire match. $1 can pay toward your 6% mortgage or be doubled on deposit to $2 in your 401(k), if available. And pay off all high interest debt first. This should stand to reason, but I've seen people keep their 18% card debt while prepaying their mortgage.