Did she give you an invitation? If so, then put 500 baht or so in the same envelope (minus the invitation) and give it back to her. Apologize you are not able to make it. If she did not give you an invitation then you are not obliged to do this. This method is more common than practical gifts for the house. For more information about what to wear, and what to do if you are invited to a wedding, then please visit my Thai Wedding Blogs.