W/O a lot of slide rule calculus and based on actually shooting one, I'd say 100-125 yards with 240-250 grain 'Ruger Loads' and about half that with garden-variety 45 Colt loads using SWC's or expanding bullets. In reality there are other factors, not the least of which is the coarse sights often found on lever actions chambered for wheelgun rounds.
A good plan is to staple up a 7" paper picnic bowl and let her shoot at it from field positions, with the load she is going to hunt with. Wherever she can keep 90% of her shots in the middle of that bowl, is about the farthest she should hunt with it.
“Nine-tenths of tactics are certain, and taught in books: but the irrational tenth is like the kingfisher flashing across the pool, and that is the test of generals. It can only be ensured by instinct, sharpened by thought practicing the stroke so often that at the crisis it is as natural as a reflex.” ~ T. E. Lawrence