River did try to keep her amusement over his song of choice hidden. After all, she couldn't give her position away. Though in peeking around the edge of the dumpster, River kept an eye on what Jack was bringing with him. The two demons were easily spotted, but she hadn't exactly been expecting the zombies. Oh well, they would go down as well. River was fairly confident that Jack could take care of the zombies while River exorcised the two demons.
Pulling herself back behind the dumpster, River waited a few moments before she elbowed the dumpster. Thankfully, the thing had been emptied, or was mostly empty so it made a resounding noise so it drew the demons' attention and brought them further down the alley towards the trap. She well knew this could end up very badly for them, especially with the no powers thing, but River was going to go down fighting and trying to lessen the attack on the complex any way she could.
Thankfully, both demons stepped into the Devil's Trap unknowingly. Once they did, River leapt out from her hiding place, drawing one gun and pointing it at the demons.
"Hello, sweeties. Sorry, but you can't stay." At which point she began reciting the exorcism. It wasn't her preferred way of doing away with evil, but knowing the people that were possessed were innocents, she wasn't about to go around shooting them. She kept her concentration on the exorcism and watching the demons as the black smoke began to filter out of the people's mouths and noses. It was kind of strange to watch, but River had seen plenty of strange things in her life, so this didn't phase her in the least.
Somewhere, she heard a howl, but River didn't exactly pay it any attention. She couldn't be distracted from sending two of Lucifer's minions back to their pit.