A good start is finding out wheither a “Woman Specific model” suits her, or if a small ‘mans’ bike would do the same job and be better value…
I tried my better half on a small (man’s ok Boys) HT, and messed around with stems & seat position, but she just couldn’t get on with it. So we tried the neigbours Myka, and one was purchased not long after. Now she’s much happier with the shorter TT, and more upright position.
Men’s bike ‘tend’ to be better value, components wise, so if a ‘mans’ bike fits her, go for that!
EDIT: A quality saddle, that she likes, is also a really good starting point. If she’s uncomfortable, she ain’t gunna ride it, no matter how good or expensive it is
(recommendation for specialized ‘arse measureing’ service here, and cheapish ‘right sized’ seat)