It seems that only a few Indonesian fans knew about this but I'll try to tell what I remember.
I already knew about this rumour a few days after the Indonesian concert but I wasn't able to contact the guy, so I couldn't confirm it. However, the guy finally uploaded the image of his gift for Mami 4 days ago via Kokoro no Tomo TV's twitter with the ID of @gugunpride
So basically the story started from this:
In the picture, you can clearly see Mami's and Rina's surprised face, right?
The guy in green actually gave Mami a handmade pickguard designed by himself, and the pickguard itself looks like this:
I don't know if it's true or not but according to the guy, Steve Vai also liked the design and wanted to buy it. However, he refused and said this was made only for Mami. All of those details are carved or something like that.
BTW, the guy also said the situation in the MnG session was funny because SCANDAL almost abandoned the other fans (probably because they were still surprised) and Mami were smiling very brightly towards him. Because of the gift, the fan was able to handshake Mami twice, got an intense eye contact with Rina and a high five with Tomomi.
I wonder if Mami will use it one day.