بسم الله الرحمن الرحيم
(Fatwa: 729/566/D=8/1438)
What you heard is correct i.e. if you have enough time then offer sunnah muwakkidah and if you are in hurry and on the way or in train or bus then leave it. The same ruling shall apply to the Fajr sunnah. However, if he missed sunnah muwakkidah while there was time then he shall not be sinful as sunnah is exempted during a journey. The intention of your friend would be that when there is no sin by leaving it then there is no need to offer it but it did not come into his mind yet if he offers then it shall bring reward.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband