oh dear, sorry about Doodle, but sounds like he went quick with no suffering.
I had a 16 year old bantam hen who went the same way. Let her out, she started pecking the hen food, then suddenly went wobbly and dropped by my feet - gone! I was upset as I had her since she was a chick, but was happy in a way she went so quick.
Not sure if the red mite would cause the heart attack as such, but I know hens hearts do give out at any time. I have lost a few like this, for no real reason.