Tā qiúhūn de shíhou, péngyǒu zài pángbiān dǎbiāngǔ.
When he proposed, his friends were cheering him on from the sidelines.
Tips
usage
Image from old opera: while the lead drum drives the action, the side drum adds supporting rhythm. 打边鼓 = you're not the main voice but you're adding pressure and energy to someone else's pitch.
