Not a Muslim but it's a bit iffy. In a general sense, idolatry in Islam is to deny the monotheistic nature of Allah, known as Tawhid. Tawhid holds that Allah is indivisible (unlike God in Christianity who is typically worshipped as part of the Holy Trinity) that he alone is God and that he exists wholly independent of everything else in existence. He is effectively considered beyond space and time and creation. Tawhid drives a huge number of other laws and rules in Islam - the refusal to depict Allah in art because according to Tawhid, to depict him in art is to attempt to capture something beyond human comprehension.

Denying Tawhid, in simple terms, involves elevating anything in creation to the level of Allah. I can't find anything concrete that says that claiming prophethood involves denying Tawhid, and in Islam the Shahada (declaration of faith) includes both Tawhid ("There is no God but Allah") and a declaration that Muhammad is the messenger of God - since the prophethood of Muhammad is not included within the Tawhid (and declaring the prophethood of Muhammad also tacitly declares prophethood of all those who came before like Moses and Jeremiah) it seems as though it is technically possible to proclaim that someone is a prophet without denying Tawhid, and therefore without committing idolatry.

Ahmadiyya generally get around the "Muhammad was the last prophet of Allah" part of Islam with a loophole - they separate prophets into two groups and say that Muhammad was the last prophet of group A, the law givers (like Muhammad and Moses, prophets who deliver a new set of rules from God). They think that there can be more prophets from group B however, which is basically the prophets who come to clarify prior sets of laws like Isaac or Mizra Ahmad.

I can elaborate more based on my understanding of Islam if you want but it'll probably be a bit long.