掉 指的是 一个物体受重力作用，从一点垂直坠落 （An object drops from a point vertically under the influence of gravity ) 倒 指的是一个物体身体发生倾斜，或者重心偏移 (The object's body leans.,or its center of gravity moves) 所以，根据语境，你的朋友是正确的 You can say something in the tree 掉了下来.For example, 果子从树上掉了下来 树上的叶子掉了下来 And say something leaning against the tree on the ground 倒了For example, 靠着树的梯子倒了 倚着树的竹竿倒了

So, how to use them? 掉：when an object does not touch another object and far away each other. 我的语文书从桌子上掉下来了。（从桌子上掉到地面或地毯上） 我用手接住了树上掉下来的一个苹果。😜😜 小心！屋顶有东西往下掉。 And these: It's usually followed by a action verb. 怎么办？我的钱掉了！😭😭 Here it means the money is lost. Here means run out: 我的苹果被他吃掉了。 所有的钱都被花掉了。 倒：is usually used to indicate that an object falls down on the other object's surface. When it means that it's usually followed by a action verb. 她在地铁里晕倒了。 一颗子弹射入了他胸膛，他捂着伤口慢慢地倒下。 他不小心被石头拌了下，摔倒了。 Edited by Bruce 🐈🐈

掉 = throw. fling down, needs an external actor to initiate the action. 巨人把树掉过去了. the giant flung the tree over there. 倒 = fall down (usually on its own accord) 树被倒下来了, The tree was fallen. By what means we don't know.

1、一棵树掉下来了。 2、一棵树倒了。 两个句子都是正确的，就看你的语境选用对应的句子。

谢谢 大家