My PH was very happy with my choice of a 375H&H with 300 grain SAF's.
Took mine at 125 yards and about 3" behind the right chevron.
I got the right lung, but not the left one.
Ran about 100 yards pouring blood from the right side.
The exit wound is visible in this photo in a black stripe, 2 back from the chevron.
I was upset with myself on the poor shot at such close range.
But was thankful for a quick recovery, mostly due to using a 375 and SAF's.
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I will emphasize that hitting the "chevron" is the most important.
Having the power of a 375 to reach the other side is important as well.
As for a minimum, Dr Kevin Robertson recommends .30 calibers or 7mmRM.
But you don't loose points for going above that.
If you have time to work your your daughter up to a 30-06, you should consider it.
Lots of 22lr practice with a few shots of the 6.5 for starters.
Slowly move up as her confidence grows.
As others have stated, the addition of a recoil reducer could be in order.
My wife started with a 270WIN and in less than a year was ready for a step up.
It was too close to our safari to be making a change, so we used the 270.
Now she's shooting a 300WM and 375H&H, quite a step up for her.
Slow and consistent practice is what got her there.
BTW - My wife is about 64" tall and 130#.
Got mine with a 225 grain Woodleigh PPSP at120 yards smashed the near shoulder shredded both lungs broke offside shoulder and lodged under the skin on the offside.