I think having fun is in line with Thai traditions, so I don't think it's against traditional ways. my guess is that they were or are reluctant to celebrate birthdays and especially give/receive presents because that focuses on individuals and puts one person in the centre of attention, instead of being more of a community and sharing occasion. as it was mentioned above, Thais tend to invite others to celebrate instead of being invited by friends, that sounds to me like a local adaptation of birthdays as we know them.
I want to know what would be an appropriate gift for her birthday.