Does his speakers have separate binding posts to send signal to different parts of the speaker? For instance, many speakers have four binding posts in the back. A positive and negative for the woofers, and a positive and negative for the midrange and tweeters. If this is the case, then your friend is bi-amping his speakers. Typically one would want to ensure proper level matching through independent volume controls, or at least one should be able to gain match one amp to the other.
if your friend is using two amplifiers to hook up to the same set of binding posts, he shouldn’t as it could surge the amps, his speakers, and potentially his circuit/house.