いいお友達でいてくれてありがとう。 ii otomodachi de itekurete arigatou. ii　(good) (or yoi （良い）is more correct　in term of grammar, perhaps). otomodachi (friend) (you can say less formal "tomodachi" also.) de (just a connective, to make the sentence correct in grammar, has no meaning) itekurete (roughly "for being") arigatou (thank you). (it's probably not necessary to say "arigatou gozaimasu" to make it formal since you're talking to a friend). I hope this helps!