What you are saying is true but I would caution that if he buys more power as you suggested earlier, he may also kill his tweeters and/or even the woofers. There is a limit how loud those speakers can play and that limit depends a lot on his room acoustic condition and the SPL he needs. I think an average power of 200WPC (sustained) will damage his speakers. For peaks, it is hard to say, as it depends on the contents, magnitude, and duration of the peaks.
Practically speaking and simply put, it is safe if there is no audible distortion.
If he launches his speakers because he exceeds the ratings/runs it hard for too long, that's not the manufacturer's fault, nor is it the fault of the equipment. However, he asked if it's safe to run his receiver at -0dB and at 140W, I would say "NO". If he adds power (enough to actually make an audible difference), he's on his own but severe overpowering usually results in more blown woofers than tweeters, assuming the drivers were chosen with a margin of safety WRT power.
Distortion isn't audible until it reaches whole percentage numbers and human ears would have a very hard time distinguishing 10% THD from the music at with an average setting of -0dB. If the speakers run into thermal compression, the speakers won't be long for this world.
Guitar/bass players and PA system designers know how to achieve extremely high SPL without component failure- add power AND speakers. One Watt into one speaker produces a certain SPL but if you add another speaker with its own amplifier that's putting out 1W, the result is 6dB higher, where just doubling the power results in a 3dB increase (on average). Since one channel of amplification won't appreciate a second speaker connected parallel to the first (unless the impedance is high enough that the new load isn't a problem), operating at -0dB may not be a problem and it will produce more SPL. Unfortunately, most receivers aren't made to be run hard and put away wet, as I mentioned. Using a separate amp with another pair of speakers, with signal from the preamp outs, would yield the best results, IMO.