Sleepovers

Found out that not only did my boys go to a movie and bowling with the girlfriend this weekend, when the power went out at their father's (on account of the ice storm), they spent the night at her house. Is this just to be expected right? Nothing I should be upset about? That's just how it's going to be?!? Do I say nothing? Do I send a simple text saying, "if you are ever in the situation again where your power goes out, please see if I'm available to take the boys in trade for another night rather than taking them to her house." Or is that not my place as a co-parent in this new fucking unbelievable paradigm?