I was playing Rengar with a Shen and cho, along with Anubis and cait, when after the game starts both Shen and cho declare they aren't tanking. Frustrated but team oriented I start building tank Rengar and wasn't that disappointed. The enemy team was mainly melee , and ranged ad with a hybrid tank Garen.

I built phage
Ninja tank
Thornmail
Sunfire and then was going to build spirit visage.


What do you do in these situations and how was build considering the problem?

I wouldn't worry. Shen and Cho are naturally tanky. I would continue building tanky DPS and hope that our combined innate tankiness and decent damage output would overwhelm the opponent.

If the enemy team is all AD, then yes Thornmail is an excellent call as well. From what it sounds like, you're probably not even at level 30 yet and neither team had anything resembling a decent team comp.