I want to give Ayahuasca to my friend.
He weights 90 kg, and I 60 kg.
Does that mean that I need to give him 1.5 of the dosage that I take?
By the way I take about 2.5 gr raw Syrian rue + 2.5 gr raw Mimosa (both powdered, uncooked), and I get full experience, by this recipe https://www.youtube.com/watch?v=ewamkPjI_pg

Theorethically yes, but i am not sure if it works that way.
I have seen huge guys (120kg+) drink one cup of Aya and purging for hours while i(75kg) drunk 2 cups of the same batch and laughed it off.

Your dosage sounds reasonable to me, i would give him the same amount and ask him if he likes more 45 min after the ingestion.
Since its an Aya analgoge you can always increase the dosage.
And a slower more gentle transition might be more enjoyable for a beginner.

I found out, that my friend have already taken Ayahuaska once about 2 months ago.
I gave him about 4 gr syrian rue + 4 gr mimosa, since I felt that was the dosage the plants
wanted.
He said it was strong but he enjoyed it.

How did you serve the rue? tea or did he swallow whole seeds?
If you served tea how did you prepare it?