People have a responsibility to manage their mental health problems to the best of their ability.

Sometimes as someone with bipolar disorder type 1 with psychotic features, I can become extremely enraged and paranoid and feel very violent.

That doesn't make it acceptable for me to actually haul off and attack someone. In those times I need to isolate, and when I am able to get treatment, I need to do so. Only in such a scenario in which I could not avoid someone, could not access medication and am having a severe episode, could it justifiably be said that I could not help myself if I hauled off and hit someone.

If your 'friend' cannot stop themself from setting things on fire, then they have a responsibility to not buy lighters/matches/etc, to not go into people's houses knowing that they might set something on fire, etc. They have a responsibility to keep others safe until they can get proper treatment.

Lighting your house on fire out of malice and lighting your house on fire because they don't bother to take precautions to protect others from their disorder, are not any different at the bottom line. The bottom line is that they made no effort to protect you from themself because they're not really your friend.