The conjecture is false.
Explanation:
Let m∠B = 60°.
Since ∠B and ∠A are supplementary, they add to 180, this means that
m∠A = 180-60 = 120°.
Since ∠B and ∠C are complementary, they add to 90, this means that
∠C = 90-60 = 30°.
If ∠A and ∠C are complementary, then they add to 90°:
120+30=150
This is not true.