Standpipe will be fine, it's not that that is the issue. If the pipework blocks or constricts downstream of the shower, then the washing machine will happily continue pumping out, only for the water to emerge in the shower tray... To prevent this happening, any NRV (or a HepVo valve) needs to go on the shower waste, before it joins the main run.
I went out to look at a job once, young lady had run herself a rather full bath, enjoyed her soak and pulled the plug, before going into the adjacent bedroom to dress. She came back out to a soaked bathroom floor, and water dripping through the ceiling in the kitchen below. Simply, the water had very quickly found its own level. Being unable to get away fast enough, it had reappeared in the shower, rising up, and overflowing the tray. I told her at minimum, the pipe needed cleaning out, and ideally, bath and shower putting on separate wastes to the stack, to eliminate the problem recurring.
I would also seriously think about putting some access points in the run, with removable panels in the floor above, for future needs.
I went out to look at a job once, young lady had run herself a rather full bath, enjoyed her soak and pulled the plug, before going into the adjacent bedroom to dress. She came back out to a soaked bathroom floor, and water dripping through the ceiling in the kitchen below. Simply, the water had very quickly found its own level. Being unable to get away fast enough, it had reappeared in the shower, rising up, and overflowing the tray. I told her at minimum, the pipe needed cleaning out, and ideally, bath and shower putting on separate wastes to the stack, to eliminate the problem recurring.
I would also seriously think about putting some access points in the run, with removable panels in the floor above, for future needs.