Learn Italian: Le sue guance sono gonfie e morbide. - His cheeks are swollen and soft.

This follows the rule for feminine nouns ending in an unstressed -cia or -gia. If the letter right before it is a consonant (like the n in guancia), the i is dropped in the plural. If it were a vowel (like in camicia), it would keep the i (camicie).

The translation says "His cheeks", but couldn't Le sue guance also mean "Her cheeks"?

Yes, absolutely! In Italian, possessive adjectives like sue agree with the thing being owned (the feminine plural guance), not the gender of the owner. Without extra context, this sentence could mean "His cheeks", "Her cheeks", or even formal "Your cheeks".

Since gonfie comes from the dictionary form gonfio, how does it become gonfie?

Gonfio is a regular adjective, so it changes its ending to match the noun. It becomes gonfia for feminine singular, and gonfie to match the feminine plural noun guance.

Does morbide mean "morbid"?