The right amount of carbs is often debated on this forum, with views ranging from carbs being the spawn of Satan to carry on as normal.
As with most things, it basically boils down to moderation.
I recall when I was dx'd it would have done my head in if I was told I had to radically alter my diet, and I think having just been told they will now have to inject every day for the rest of their life, it puts unneeded pressure on newly dx'd to suggest that they can forget about toast as well.
Some accomodations are sensibly made. For example, I'm not going to sit down and polish off a big pile of cake anytime soon, I'll have salted popcorn instead of sweet if I go to the movies, I choose diet drinks.
But if I decide to have a full fried breakfast, I'll have the works, including toast, potato scone, hash brown, whereas some will say, oh no, you can only have the bacon and egg. I can eat the whole thing because I can make a sensible guess at the amount of carbs in it from Carbs & Cals app on my phone and past experience tells me how much insulin is needed to cover it and remain relatively flat. Your mum will pick up that experience as time goes by.
Some do find that a low carb approach works for them and fair do's to them. But I think newly dx'd are as well to defer making those sort of decisions till they have learned more and can make an informed decision.
Although it is a huge shock to start with and it seems very grim at the moment, as time goes by you and she will be surprised by how unrestricted it actually is. I've been backpacking all over the world, eaten local dishes wherever I've gone and not had any issues even with relatively high carb meals.
It does take a bit more planning ahead than your non-T1, but seeing as your mum is a parent she's likely already got skills in that department from raising you!
Oh, and cake isn't totally out the window either! I haven't actually got that much of a sweet tooth, but I don't say no to a bitbof cheesecake or eclair every now and then, but would tend to steer clear of heavily iced stuff, mainly because the sheer amount of insulin to cover the sugar in that coyld have unpredictable results if you get the calculations wrong.
Good luck to your mam - it's not a walk in the park, but it is manageable!