She will be absolutely fine, I guarantee it. I'm T1 and have been for over 40 years and I am only rarely stopped or asked about my stuff at security. I have never been asked for proof of my condition or a Dr.'s letter.
Using a pump she should declare it at security, since she will be wearing it through the scanner. I don't even mention my condition (I don't use a pump) or take my supplies out of my carryon.
All medical supplies are exempt from the liquids rule, she can carry on as much as she wants. I use cooling wallets (
Frio - Insulin Cooling Wallets) which are water soaked gel which use evaporation to cool, and these are accepted without problem too.
Basic rules, have some form of ID; take pleanty of supplies (remember that icelandic volcano); keep them cool (not cold!); have some glucose, preferably in solid form (if you are goint to have problems it will be with things like juice); and enjoy your flight.
For a trip crossing more than a couple of time zones she should discuss her needs with a professional. Not complicated, particularly with a pump, but she should know what to do.