I do not understand that.

Why should the "strong" friend be overwhelmed?

The strong friend is usually one who shows strength on the outside and hides the issues going on inside and in their life.

I think the implication is that the "strength," while not exactly a facade, is something that can hide the overwhelm and other stressors that are going on under the surface.

sometimes the strong friend is always there for their friends but doesn't invite anyone in when they themselves are in crisis