Question about leaf ash fertilizer

[IrishAyes]

While I was away 2 weeks a neighbor burned his leaf pile; he offered the ashes to me for my roses, but someone else told me he uses a bit of gasoline to start the fire, which he denies because it's illegal. My question is, if he did start his leaf burn with a little gasoline, would the harmful elements remain in the ashes? Would they be safe to put under my roses? It's easy enough to say don't bother, but I don't want to be Nervous Nellie either. I always have lots of leaves of my own to burn; still there's never more than enough for 20 big rose bushes not to mention all the other stuff like young trees etc. I want to be careful but not ignorant.