The latter is actually the perfect setup, for both frog and spider. As the spider feasts on prey, discarded remains begin to pile up. This (along with the spider's appetizing eggs) attracts ants and other hungry pests, which the tiny amphibian eats up before any damage can be done to the nest. In exchange, the spider does not kill and eat the frog. That's ... pretty much it. These two radically different species have come to an unspoken understanding.
Or maybe a spoken one. We don't speak spider-frog.
Of course, this is a giant, angry, hungry spider, and sometimes they forget themselves. Should Xenesthis ever lose its shit like Oscar Madison on bath salts and start picturing its pet Felix roasted on a plate with an apple in its mouth, the suddenly-not-so-harmless froggy will unveil its secret weapon: toxic skin. Biting into it isn't enough to kill the spider, but will certainly persuade it to cancel its dinner plans and go back to chewing on something safer. Like a cobra.