If you are buying the sled so she can ride with you and you are riding in the mountains it matters what you ride. Although not published on the Polaris website there is usually a considerable difference in horsepower between a 600 cc. sled and an 800 cc sled. If you ride a bigger sled with a longer track even if she is small with a smaller motor and a shorter track she will likley not be able to go into all the places where you can go if you have a bigger sled with a longer track. This means either you will not be able to ride in certain areas you would like to go as you have to hang back with her and you will be pissed off or you will go into areas she cannot get into leaving her behind and then she will be pissed off. If you are riding together and both riding 600 - 155's you are good to go but if you are riding together and you are riding an 800-163 and she is riding a 600-155 I see some potential conflict. If you really like her and you plan on riding together I suggest you ride similar sleds.
Or, she could just poach his line after he goes somewhere...