Is it permissible for a person to eat a specific amount of protein daily to maintain their health, or is that contrary to what is better, and should they emulate the predecessors in their asceticism regarding food?

Your friend is mistaken. Allah has permitted eating from good things, and the Prophet, peace and blessings be upon him, ate from good things. He would not refuse what was available nor seek what was absent. Asceticism is not about reducing good things. Rather, a Muslim should eat what benefits him and thank Allah for His blessings. Ibn al-Qayyim stated that the Prophet's guidance concerning food was not to refuse what was available or to go to trouble for what was absent. Eating little is blameworthy if it incapacitates one from worship and good deeds. In conclusion, you may eat and drink as you wish without extravagance, and you must thank Allah for His blessings.