The REAL answer is "it depends."

For example, if there's a trick attack window and you use TBN and pop it within that same duration to use a DA'd Bloodspiller, TBN is officially a gain. Another example would be is if you KNOW your GCDs always end up a certain way during a fight. If you've been noticing you always finish a phase on a hard slash, deliberately using TBN to use bloodspiller instead is now an official gain.

Then there are instances where using TBN may be a DPS loss, but a necessary one to avoid a bigger loss. If you used BW and delirium, you might find you're swimming in so much MP you're at risk of capping, and using DA isn't enough. TBN on whoever's MTing will then be a good solution, better than using DP (a 14 pot loss) or even overflowing, which is huggee potency loss. Of course, there's also the argument that using TBN might save your healer a GCD, especially during certain segments where damage is heavy on the tanks.

Personally, I use the hell out of TBN and I achieved OVERALL ORANGES! If your buddy's worried about his parses, he should focus on fundamentals first and min-maxing later, unless he's already at high 99's across the board but I doubt it if he thinks TBN is as simple as "it's a loss."