Can you use the `Retreat` card to return an opponent's unit to their hand, or is it only your units?

No, you cannot use Retreat to return an opponent's unit to their hand.

According to the card text, Retreat specifically targets a "friendly unit." As established in Riftbound rules and clarified in RiftJudge FAQs (such as FAQ #3364 and #5292), the term "friendly unit" refers only to units controlled by you or your teammate in a 2v2 game. It cannot target an opponent's unit.