I was reading the current issue of buckmasters magazine and noticed an article about blind hunters. I saw that some guys have had special mounts machined for their rifles to allow a seing hunter to look over their shoulder to help them sight.
I designed a similar system for a friend of mine who's legally blind a few years back. the nice thing about my design is it uses all existing parts so you don't have to get anything machined.
I purchase a 3 sided pictanny rail as used for an M4 flat top to allow it to carry multiple sight systems. this will mount directly to standard weaver single piece base. then you use weaver style extra high steel rings and mount them to the side of your shooting hand and put a pistol scope in it. you can also mount a scope on top if it helps you too see as well. the pistol scope allows your assistant to line the rifle up for you.
if you purchase steel rings and a base from cheaper than dirt the whole system minus the scope only costs about $60 and is premachined from parts you can purchase from a local gun store.
this system works best on a break action rifle, semi auto or a lever gun. it can be mounted on a bolt action as long as the bolt is 'off handed' to the shooter (leftie rifle for a righty and vis a vis), otherwise the side scope interferes with the bolt.
if you sight in to 2" high at 100 and dead on side to side you can hit in the kill zone to about 200 yds with this system. if you sight in 1" to the strong side at 100 yds you can hit to about 275 yds.
feel free to use any information here if it will help you. contact me if you need details. btw, it also works on crossbows.