Parents usually open this question with which exam their child would do best in. That is a reasonable instinct and it is the second question. Asked first, it produces a choice that does not lead anywhere in particular.
The first question is the destination
The United States, the United Kingdom, France, Turkey and the Netherlands do not read the same documents. An application to a British university is built on A Level or IB; a French one runs through the Bac and Parcoursup; an American one usually pairs a diploma with SAT.
Once the destination is named, the list of sensible routes usually shrinks to one or two, before ability is discussed at all.
The second question is what the school offers
A school teaches what it teaches. Where a school runs the IB and not AP, choosing AP means preparing outside school, which is entirely possible and is a different amount of work.
These two questions together generally reduce the options to two, which is a manageable decision. Starting from the child's strengths keeps all options open and makes the decision harder rather than easier.
When to decide
Mid secondary school, before the point where subject selection locks the route in. In most systems that means around ages fourteen to sixteen.
Deciding later is possible but expensive, because content does not transfer between systems and a year of it can be lost.
When the child cannot decide
This is common and is not a problem to be solved by pressing for an answer. Where the destination is genuinely open, the route that keeps the most doors open is the better default, even at some cost in workload.
A choice made under pressure at fifteen and regretted at seventeen costs more than a slightly heavier programme.
Looking for the safe option
There is no route without risk, and a route chosen because it looks easier often turns out heavier in a different way. What reduces risk is not the choice of system but a realistic calendar and an early start.
The parent's role
The useful contribution is the destination question and the calendar, both of which a fifteen year old cannot reasonably manage alone. The subject level detail belongs with the school counsellor and the student.