I was glad that I happened to see the driver I'd know well.
"顔見知りの運転手と偶然お会いできて嬉しかったです!
嬉しかった I was glad that ..., I was happy thatなど
偶然会う happen to see, happen to meet,
顔見知りの運転手 the driver I've known well
ここでは述語動詞の時制を過去形としておりますので、the driverの後のhave knownを
時制合わせで過去完了had knownとしてます。
I was glad that I happened to see the driver I'd know well.