After Frux's father (who is nearly feral) tells her not to come back we jump to her with Muts (separated by a cell) and she says "I think I lost my father today."

During this conversation when Muts defends Wrex (he just wants to protect everyone) Frux tires to explain to him that he never even mentions Muts.

When Frux comments about Muts being locked in a cage he abruptly opens it for her.

Maybe we seen that Muts has already told Wrex about his 'new friend' during a previous encounter.