A friend bought his girlfriend a .260 but wished it had been a 7mm 08. But she is not a lightweight so your fiancee would be happier with the .260. Or a .243 which will do nearly anything you need to do to a deer.
Do you think she might be interested enough to want Her Own Gun?
This sharing of firearms with men seldom works out well for women.
Especially if she is small statured and would benefit from a Junior model or a shortened stock.