Roland Microcube guitar amp damaged beyond repair?
According to the manufacturer , the Microcube takes 9VDC input. 

From what you described, your friend took a circuit expecting 9VDC and wired it directly into 120VAC (assuming you're in North America).

If this is the case, I don't need to rely on my >10 years in power supplies to tell you that this is a VERY BAD THING. Your friend has almost certainly destroyed the low-voltage electronics inside the amplifier by applying line voltage directly to a low-voltage DC circuit.

Properly-sized fuses blow as a consequence of something bad happening. I would expect that the AC application caused a short-circuit which opened the fuse.

If you intend to replace the fuse and try your luck again, you MUST ensure that you're using a 9VDC supply (preferably current-limited to less than 1A) to power the circuit. Otherwise, you'll simply pop the fuse again.