That depends on the exact nature of the trade between the two nations and is not necessarily true.

Often you have the situation that one country supplies a certain number of goods while the other one is stronger in another group of goods. Examples are Argentina-Brazil, US-Europe, Europe-China. Thus, if in such a constellation country A is having a decline, it will also affect the demand from goods from country B, and thus it is not necessarily positive.

Only if there is much overlap between the goods produced by both countries, and at least one of the countries isn't really importing goods from the other one (and thus doesn't affect demand on their goods) then there may be an opportunity for a country to capitalize from others' decline.

In general, often the general rule is the opposite: the stronger all countries are, the better the opportunities for a single (export-oriented) country to also improve.

This does not mean that the thread title is incorrect, but the OP's view is far too simplistic.