Tyonee, You don't state your granddaughter's age, but bad or dysfunctional behavior is not acceptable for any person regardless of whether they're on the autistic spectrum or not.
Of course there are differences between a person who is aspie versus someone who is not on the spectrum. The aspie will have some special needs & requirements that their loved ones should accommodate in order to help them achieve their full potential, and lead a peaceful & happy life. However, being autistic - especially high functioning - is NOT be a free pass or excuse for taking advantage of or mistreating others.
I surmise that your granddaughter's behavior reflects immaturity, as opposed to being a general trait of Aspergers which it's not. Children often use crying to manipulate their parents & others, & immature teens/young adults will do the same if they have learned through poor parenting that it will succeed. I'm not sure what forum you saw "adult aspies brag" about using their Aspergers to manipulate, but those would be immature adults, & not reflective of the kind of person you desire your granddaughter to grow & mature into. As an aside, I personally don't see the benefit of following such a forum (or group of people) in your search of further knowledge & understanding of ASD.
I understand & empathize with your feelings of frustration, anger, disappointment & hurt. Your reaction may also reflect a cumulative build up of emotions. You're in an especially difficult position because although you obviously love your granddaughter a great deal, are very devoted to her, & are an important presence in her life; you do not hold the "in charge" parental role.
Who knows about the overall family dynamics? I would think her mom should have provided non-judgmental emotional support & love to her daughter, but then circled back to you to find out what happened. You state that you 'work together over her' so IMO her mom should have also been emotionally supportive to you, AND discussed with you what (if anything) needed to be addressed with your granddaughter. It is possible (or not) that her mom is slightly jealous of your closeness & valued the opportunity to be the "one" to whom your granddaughter turned. Possibly mom is a pushover for tears. Maybe the reaction was simply an outlier - we're all only human, & her parents are probably equally drained & exhausted. But in any case, you cannot control how her parents react or handle things. You can only
try to positively influence your Granddaughter's development & behavior, & provide the best role model you can be for her.
I think you were absolutely right to inform your granddaughter what is acceptable, & that you will not tolerate bad behavior (including lying & fabrications). If you actually yelled at her or reacted angrily, (if it were me) I would apologize to her specifically for that, & explain why you were so upset. I would also explain your feelings to her so that she can know WHY what she did was not a good thing. Do it when you are calm, & be careful to not come across as critical or judgmental. Try to instead discuss it in a very loving & caring way so that she is reminded of your love for her, & learns that this is how mature adults behave & communicate (i.e.; openly & honestly).
As an entirely separate thought; is it possible that your granddaughter was seemingly happily watching a movie but internally was upset so was texting you about her distress (to vent or receive attention from you) which unintentionally backfired when you contacted her mom out of legitimate concern? But then your granddaughter did not want to share her real feelings with mom so replied, 'what crisis?' ... & ended up genuinely upset & crying (versus crying to manipulate) when you (her beloved & normally compassionate grandmother) got angry/upset? By the way, this is NOT to blame you for what transpired or for getting angry. (We all have our breaking point!) Just wondering if this could be what occurred instead of your granddaughter just creating drama or inventing a crisis for attention.
Either way, I would talk over this incident with your granddaughter when you are both calm, & also ask her to try to explain why she thinks she might have done what she did (i.e.; texting about a crisis when she later claimed there was none). I personally would let drop & not follow up with her about her crying to mom that you were "mean". If she is resistant or starts to get upset or anxious, I would not insist or push, but would instead ask her to please come to you when she is ready because you want the two of you to understand each other better, & also to repair your relationship after what was an upsetting interaction for you both. Also, ask if she would prefer to instant message or email this 'conversation' if that is easier for her than a verbal discussion.
Good luck with things with your granddaughter. I think she is lucky to have an extended family who care as much as you do.
Here are some articles you may want to read if you haven't already.
Temple Grandin On The Secret To Success For Kids With Autism
Autism advocate Temple Grandin: 'Old-fashioned '50s parenting' can help kids | MLive.com
My Aspergers Child: The Damage Done: Over-Indulging the Aspergers Child
PS - It can be very cathartic to vent, but since the majority of AC members are on the spectrum I think they can better relate to seeing things from your granddaughter's perspective. That is great for gaining insight, but a parenting forum for adult family members of autistics might be a better place for you to receive the support, compassion, & understanding you also need (& deserve).