First of all, congratulations to your classmate for the successful proposal. The key to your classmate's success is the gift, although you also played a role. Haha, just kidding, your classmate's success is inseparable from the relationship between you and your group. I really envy your roommate.
As for gifts, you can think about their married life and give them some household appliances or something they use now, such as a pair of engraved necklaces, or a pair of life-like dolls for the two of them, or a box of blessing chocolates for the two of them. These are all good choices. I hope you will be satisfied with my answer.