Difficult to say for sure as it depends on the tour operator but I would expect a reminder would be sent on or the day after it was due giving you a nudge and 7 days to pay. I doubt very much if any tour op would cancel without sending you a reminder letter but if she's going to pay straight after crimbo maybe a phone call to let them know would be sensible . I'm sure they would give her a few extra days.
I know if you book through a travel agent , thomson shop or similar they ask for payment a couple of weeks before it is due to allow time for them to pass the payment on . This is why it would seem some people have a due date 6 weeks before while other have 8 weeks.

It's not unusual for me to pay a couple of days late as I try to move into the next months credit card bill or simply forget and I would normally get a reminder letter a few days later that crosses with my payment .

Thanks lynwestie for your reply, just to be on the safe side I am going to pay it for them for now.

Our friends booked a flight only through a travel agent and he forget to pay the balance on time. First thing he knew was when he got a letter saying his flights had been cancelled. He had no reminder either in writing or over the phone. This was not a small independent but one of 'the big two'.
I think your plan of paying it for them is a good idea.

.right off to check the date when my flight only booking with Travel republic is due

IFF you don't pay you second deposit on your holiday will the company automatically cancel your holiday thanks

Depends upon the circumstances but anyone who finds difficulties needs to contact the Tour Operator/Agent ASAP. If it just a matter of days delay then they may be understanding and give you a bit more time.

Some years ago my wife needed an operation but we were having problems getting the actual date. We contacted the TO (Thomsons in this case) and they allowed us an extra two weeks to sort it out and said that we could move the dates. In the event a call to the surgeons secretary brought the date forward.

One danger is you could become liable for extra charges even if they cancel for non payment. For example if you have paid a low deposit then if you do not make the payment when due not only will they cancel the holiday but you are also liable for the balance of the deposit.

This also applies if paying the balance of the holiday although you would incur extra charges for cancellation as laid down in the T&Cs. If it is due to illness/redundancy then you would possibly be looking at claiming against your holiday insurance, gain you should be talking to the TO ASAP to minimise charges