that the used water from the washer is taken from the house?
You know, sometimes this pipe collects lint from washings even if your washer has a trap. Maybe the previous washer did not. Anyway, this stuff can collect in the P section of the drain pipe and cause restrictions and therefore overflows.
This needs to be checked periodically.
Now, if this is the first time and you have fixed the problem, then you might be OK. If you allow this to vent and air out you might not have any structural damage at all.
You might want to remove a straight cut section of dry wall so that it can dry out inside and then spray some bleach in that area for mold retardant.
Now, for proper install of both units, make sure that they are level, adjust the legs. Then install that metal no burst flex hose. Make sure, make sure, that washers are installed in both ends.
Tighten up securely and if you want wrap some teflon tape around the threaded or male ends for added security.