There is a big difference between a 154 and a 149!!
Just for a comparison, I am 5'0", weigh 105, and ride a 141. (Of course, I like a shorter, flexier board, so I'm not recommending she go that small). My first board was a 143, which was a perfect size for me to learn on. It seems like a 145 would be closer to an ideal size for your lady's beginner all-mountain board, so I vote strongly against increasing the size from what she already rides, especially to 154. Has she demoed different board sizes? Does she know that she prefers a longer board over a shorter one? It is silly to buy a board that long just to save some money if it ends up being the wrong board, thus one you'll have to sell and replace.
Remember that you do not measure board size according to where it hits your face (chin, nose, etc.) Look at her WEIGHT. A longer board will feel stiffer and will be harder to control than a shorter board, but will improve stability.
What board is she on now and what is she considering buying?
Yeah I knew that gauging where the board stands in relation to you isn't a proper gauge but weight is the correct measurement. I have talked her outta the 154.
All she has ridden thus far is a 149 unfortunately which have been rentals. I have actually spent the last 3 hours researching and searching boards showing her alternatives. She was stuck on the one design for a while (which was the 154 and is why I asked) until I started showing her more designs. Also, like you said, I don't want her to get the wrong board just to save money. So with that, I am going to help her pay for it so she can get a better sized board for her to learn on. I think this next weekend I'll have her try out a 144 and a 147 and see how she does. Then make a decision. We are going to Colorado in March and I want her to be comfy on her own board by then.
I am still not that educated on what girls boards are good etc. What do you think of the Technine Dime Black as far as quality etc. She really likes how it looks and it seems to be a quality board. They also have a 144 and a 147 actually which I am thinking the 147 would suit her well.
Her biggest issue right now is confidence, once she gets through that she will really pick this up fast. She does much better than she thinks and is just still very nervous and it's all in her head. One big issue is the guys that run into her. Unfortunately, the last 2 times we were out someone has ran into her from behind and now that has her gun shy. I'm trying to get her off the beginner hill for that reason actually. Whats great though is she wants to be out there just as much as I do. hah
Thanks for the advice everyone! Much appreciated!