Last night I lent my Marshall JCM 2000 DSL/1960A combination to my friend for practice. I failed to notice that he was using an instrument cable in place of a speaker cable. Practice lasted about 1&1/2 hours. I'm not sure if he matched ohms either. Needless to say I won't be lending my amps anymore.
It'll take a little top end off of the signal--the longer the cable, the less top. I've always been told that it loads the amplifier down inappropriately, which is why it's a thing to be avoided. An hour and a half, though--I really can't imagine that doing any damage given that I've seen people do that night in and night out for year after year without any harm coming to their amps. They've been surprised when I pointed it out to them and astonished at the improvement in their sound when they started using the correct cable.
As I'm thinking about this, a faint bell is going off at the back of my worthless little brain: Is it more of a problem if one does that to a solid-state amp? I *think* that might be the case . . ..
Anyway, relax: Your friend may not be well-educated regarding cable types, but he didn't break anything of yours either.