i want to visit my foreign friend studying in china. is her invitation letter valid?

Madhu, I think an invitation letter from her is not enough if you hold Sri Lanka passport. As far as I know, you need to get an invitation letter from Chinese Tourist Bureau and a letter from a local travel agency.
Of course, if the embassy just requires an invitation letter from a Chinese resident, I think an invitation letter from your friend can work because she needs to study for one year and she will have the residence permit.