I would say 55g for one; however, I'm thinking of a 7-8" BP rather than one of the massive 10-12 inchers that seem to be less common.
I've become less and less fond of 12" deep tanks and much prefer an 18" footprint especially for bulky cichlids. I love my 40g breeder tank which is more than enough room for my single 4" parrot but I think when he/she really bulks up I'll need to convert to something like a 75g. In my opinion, it's not right to keep a bulky fish in a tank where they basically swim in one dimension (left to right). To mimic the natural territory of a cichlid, you should have a tank that is 18-24" from front to back because in their natural habitats a cichld chooses a territory with a circular radius around it, not a long rectangle haha.
If your friend thinks 30g is enough, he's obviously never seen how big adult BPs can get.
I've become less and less fond of 12" deep tanks and much prefer an 18" footprint especially for bulky cichlids. I love my 40g breeder tank which is more than enough room for my single 4" parrot but I think when he/she really bulks up I'll need to convert to something like a 75g. In my opinion, it's not right to keep a bulky fish in a tank where they basically swim in one dimension (left to right). To mimic the natural territory of a cichlid, you should have a tank that is 18-24" from front to back because in their natural habitats a cichld chooses a territory with a circular radius around it, not a long rectangle haha.
If your friend thinks 30g is enough, he's obviously never seen how big adult BPs can get.
here is a vid, also side comparo of the tank and the parrots 
