@Nira to be honest with you I thought that I read that some where in regards to the reason for which they raised her Dexterity. Because If I remember correctly it used to be a 9
The only things that you need for heavy armor is STR and proficiency. Full plate requires 15 STR to wear for example.
Heavy armor receives no bonuses from DEX, like the other armors, because it already comes(usually) with a high AC on it's own and the DEX bonuses from medium or light represent the character's ability to evade while in non heavy armor. In heavy armor you are better off blocking/glancing the blows off of your good armor.
Due to SH's now 15 DEX, she went from a -1 DEX mod to a +2. This means that when she wears medium armor, she now adds a +2 to her AC instead of subtracting a -1, and results in her having 3 more total AC. I'm thinking this is where your confusion came in. She didnt gain access to other armors, she just got much better at wearing the armor she already could.