Well, guys, if he really is talking about speaker "break-up," this would be referring to the way the speaker naturally distorts right? Which can vary depending on how much headroom the speaker allows, meaning that lower wattage speakers break up faster than higher wattage speakers.
For example, Celestion GK-100's probably won't break up because their wattage is so high, while Celestion Greenbacks are actually known for their break-up, which sounds pleasing to the ear. Both speakers are great. The GK-100's are known for great frequency response (not coloring your amp sound), while Greenbacks are just one of many popular speakers that have that nice "break-up" that people prefer.
In reference to the original question, I'm not sure which G12 speakers you are talking about. There are MANY G12's: G12M (Greenbacks), G12H-30, G12H-30 Heritage, G12-65 Heritage, G12 Century/Vintage, G12T-75's
The most popular "G12's" are listed below:
G12M "Greenbacks" - As discussed above, they break up very soon (25 Watts)...great voicing
G12H-30 - My favorite speaker, breaks up a tad bit faster than a Vintage 30, I love the voicing of this speaker. These are 30 Watts while the Vintage 30's are 60 Watts (I think)
G12T-75 - Found in most Marshall cabinets (1960 Lead/JCM Series), they have slightly scooped mids and break up LATER than Vintage 30's...these are 75 watts and MAY have been what your friend was referring to
Here's a link I found a while back that does a great job comparing the audio of some popular Celestions including Vintage 30's (V30's) and the ones mentioned above...so you can actually HEAR the difference:
http://www.bobsavage.net/bogner/bogner.htm Check out the Bogner Shiva clips
Hope this helps man, and someone correct me if I'm wrong on any of this...Later!
-Neuro