I'm sure some will mention some other bikes, but there really aren't too many electric mountain bikes with full throttle. As far as proper purpose built bikes there are a couple that come to mind. You can check out the Easy Motion Evo Jumper and the Izip Peak DS. In my opinion pedal assist is the way to go for mountain biking though. You have much more control and it's safer if you have a good system.
I've ridden a lot of bikes and for mountain biking nothing really compares to the Bosch motor system on the trail. I'm not sure if you've ridden any electric bikes, but I would recommend you try to visit a shop and try some out. You may quickly learn what defines the best.
I'm not sure if you're in the US, but keep in mind the trend is to disallow throttle on trails. I know this might bring up some controversy, but I wanted to bring it up in case you weren't aware.
It doesn't sound like that to be... what is a dirt bike? I thought they are ICE bikes with 80cc+ displacement, not a 1hp electric bike.
Are not all the Mountain Haibike, 350W Bosch systems being ridden on dirt trails? Does that make them dirt bikes?
If you have even ridden an Optibike mid drive, it is not made to be ridden on throttle alone. Why? The MBB spins at 2400 rpms and gets stepped down (like 30:1) to the chainring, 85-90 rpm. It can run down to 70 rpms under load, but it does not like it, it wants pedal assist.
the MBB mid drive is smaller in profile than a hub motor and spins about 10X faster. It tends to be more efficient, but within a smaller operating band. While the ddhub will perform from 0-350 rpms, the Opti MBB always wants to operate around 85 rpms and have you change gears. Thus, legs are needed between gear shifting and overall ride performance.
It is more satisfying to ride all MDs like a bike, as designed, and get the thrill of faster assisted mtn biking, than just throttle and change of gears.
If that's the case, then he's not looking for a zero pedal dirtbike.