You could use HandBrake to resize it to the correct (or estimated) dimensions. Maybe post an obvious screen grab and the the technical info shown in QuickTime (or better MediaInfo - App Store).I have been sent a video, but it appears stretched, how do I change the aspect ration on my iPhone, iPad or Macbook Pro please?
Did the originator edit the video? There is a "stretch to fit", "cut and pan", etc setting that can stretch clips as you place them on a custom time line (a time line/project with properties that doesn't match the source video). You want to turn that off. (in the crop tool perhaps).
Otherwise FCPX gives you a transform tool that can scale x and y independently. Dunno what other tools provide this, look for one that does more than zoom, you want to be able to scale.