the kanji 谷 can be read as "ya", like hikigaya 比企谷 hachiman, or "tani", like 米谷 kometani. i believe her name is arigaya, not arigatani, so the names should be keep consistent.
generally, pronounciations for character names are written in furigana at the magazine page margin where the characters are introduced, but i dont think this is a magazine manga. if the surname is common, a google search can help. there are websites that show how names are written