Depends.. If you do ad-hoc between two g cards, it does the max speed for g.. If you go between b and g, it will only set itself for the lowest common denominator speed (in this case b at 11). I used to have one set up as one b one g and it ran at 11, changed to two g's, and now it's about 54.
Nope, but I can see where your friend is coming from. The 802.11g specification does not include any requirement to support Ad-Hoc mode at 802.11g speeds. A company could supply a radio that only does
11Mbits/sec in ad-hoc mode and still be 802.11g compliant. I just setup several WPC54G and WMP54G PCI cards in an ad-hoc (gaming) network. Works at greater than 11Mbits/sec but I didn't bother to check how much faster.
One catch is that the WPC54GS "speed boost" technology does NOT work in Ad-Hoc mode. That's why we bought the cheaper cards.
There's no guarantee that all manufacturers will work at 802.11g speeds in ad-hoc. For example, D-Link doesn't officially support it.
I guess you're just going to have to disclose what equipment you have and deal with the issue individually by manufacturer and product.