She should be seeing a specialist diabetes antenatal team, including a diabetes consultant and a diabetes specialist midwife, at the moment.
Pregnancy makes you MUCH more prone to hypoglycaemic unawareness, when you don't know you are going hypo. This combined with doctors like me encouraging women to have near normal sugars during pregnancy is very different to what we tell women just after they have had their baby.
Literally, as soon as the placenta has been delivered, insulin requirements will plummet, so her team should discuss with her in advance how much insulin she is going to be on straight after delivery. This will be an underestimate quite deliberately and might need adjustment.
Looking after a baby with or without breastfeeding is a huge job, so it is important she keeps a really good eye on her sugars and that she doesn't worry too much about the higher ones now. (This is quite a significant change in thinking to be done compared to what they are asked to do for the previous 9 months.) She should keep hypo treatment (glucose drink or tablets) nearby and should be religious about checking sugars before driving.
You haven't mentioned what treatment she is on, so I assume, in common with most folk with T1DM during pregnancy, she is on 4 injections a day. Her diabetes team should help her with changing her dose, but she should know that she can take her fast-acting insulin AFTER a meal if she needs to. We generally say it should be taken at the beginning, but, particularly here when we are aiming for safe, not necessarily tight, control, it can be taken afterwards when she knows how she has actually eaten.
I can't think of the number of times that I've sat down for lunch then heard a crying baby, so if you are diabetic, you don't want to have taken your insulin and then not got your food.
It is perfectly possible to breastfeed with T1DM but, as with most things with diabetes, it just takes a little extra thinking. I hope she feels encouraged by your support - you sound lovely.