Wow, what a punch in the gut! I thought I was just going to browse a thread with some cool snake pics and then you dropped the bomb there.
SO's that don't love snakes as much as we do are a problem for many of us, but it sounds like you did everything you were supposed to do to educate her in advance how big Behira was going to get and it doesn't feel fair to have her change her mind like this. Meanwhile she feels like she didn't understand what she was agreeing to, whether it's because she didn't really listen (not being interested) or whatever.
I think the best thing to do right now is to give her some cooling off time and then try to have a frank discussion about it. Let her know that different snakes grow to different sizes and that Behira won't necessarily get that big. Also, she will grow slowly and you can affect the rate of growth and to an extent her ultimate size by controlling her feeding schedule.
I would start from scratch - don't make any assumptions about a comment she may have made while tired, scared, or annoyed. Give her the chance to clarify what she really wants to say.
Not sure if this helps, but I find a comparison of weight to be a better way for people to understand snake sizes because they're long noodles. A 6 foot snake sounds enormous to laymen but depending on girth it may weigh about as much as a small lap dog and helps people put things into perspective.
Also, you can let her know that she's not obligated to hold or take care of it. You're willing to do all the work and if she doesn't feel like interacting with Behira that's fine.