In school I learned that for masculine possession that noun gets a shéimiú, eg. a chara- his friend for feminine possession it doesn't, eg. a cara- her friend and for collective possession the noun gets an urú. eg. a gcara- their friend
I even looked it up on Wikipedia and got the same thing https://en.wikipedia.org/wiki/Irish_grammar#Pronouns
Duolingo has this backwards for some reason and they need to change it because what they're teaching is wrong.
You’re confusing lenition (which is what a can do with a noun that begins with a consonant) with H-prothesis (which is what a can do with a noun that begins with a vowel). They’re two different mutations; “his friend and his apple” is a chara agus a úll, and “her friend and her apple” is a cara agus a húll.