Your friend is describing people and places. Choose the correct articles based on how words sound (not how they're spelled!).

My professor is an honest person — she always tells the truth. She works at a university in London. Today the weather is perfect for a walk!

My professor is an honest person.

Use an before vowel sounds. "Honest" has a silent 'h', so it starts with the vowel sound /ɒ/. Sound beats spelling every time!

She works at a university in London.

Use a before consonant sounds. "University" starts with /juː/ — that's a consonant sound (like "you"), even though it's spelled with 'u'.

Today the weather is perfect for a walk!

Use the (definite article) when both speaker and listener know exactly what's being talked about. There's only one "weather" happening right now!