Hi Maumote Chami,
Aikido is already a complex art to study for anyone who do not have any prior MA training. To get into the politics of the various aikido organizations only intense its complexity. IMO, it is a study by itself. You need to go into the history and evolution of aikido and its respective organizations. The politics of the major organizations and the politics within these organizations and their affiliates is a never ending subject considering all (human) factors that created the politics. Sadly but realistically, rankings are tools of such politics.
We always look at Aikido as a MA created by M Ueshiba. Then, it is also argued that the term "Aikido" is also used as a classification/label by the DAI NIHON BUTOKUKAI to cover all similar MA - e.g. Korindo Aikido. Interview with Minoru Hirai sensei
Generally, the aikido we refer to and train in is the aikido of M Ueshiba and the major organizations of this lineage are:
Controlled and managed by his direct descendants - Aikikai
Of his direct students -
Yoshinkan Aikido - Gozo Shioda
Shin shin Toitsu Aikido - Koichi Tohei
Tomiki Aikido - Kenji Tomiki
Aiki Manseido - Kanshu Sunadomari
Yoseikan - Minoru Mochizuki
Tendoryu Aikido - Kenji Shimizu
and the latest addition - Shin shin Aiki Shuren-Kai - Hirohito Saito (trained as child with O Sensei)
As for the ART, you are right about, "..but at the end it's a 1 Aikido". There are different paths to go up a mountain but the view is the same at the top. I agree with Lan -
To enjoy the journey, we must remain a student and forget about the ranks and hakama.
As for your friend - do you need to correct him? IMO, no. Just ask him to do his own research rather than listening to his teacher. That 25% of the teaching may not be 100% right