Well, if she is not only your best friend but someone with whom you stamp, she's going to know if you start buying elsewhere when she sees your new stuff. I'm thinking she may not want to be pushy and that's why she doesn't always tell you when she's ordering. I'm also guessing her feelings might be hurt if that happened.
I'd address it directly with her..."Stamping buddy, PLEASE let me know whenever you're planning to place an order so I can, too, if I need something." You might also add, depending on her response, "If that's a chore and you'd rather, I'd be glad to look for someone who places orders more regularly." The latter is not said as nicely as it could be but you can refine the language so you don't hurt her feelings.
As for the specials, you could certainly find out early yourself if you sign up with the excellent $99 offer (plus the free printer's tray -- just extended thru 9/16). Otherwise, I'm pretty sure you will find news of any specials right here on SCS -- fast! I check the news on the SU! website daily and sometimes I still find out first here.
Good luck with your decision.