from django.contrib.auth.models import User
from django.http import JsonResponse


def check_user_online(request):
    if request.method == 'POST':
        user_id = request.POST.get('user_id')
        user = User.objects.filter(id=user_id).first()
        if user:
            if user in request.online_now:
                return JsonResponse({'is_online': True})
            return JsonResponse({'is_online': False})
    return JsonResponse({'is_online': None})



urlpatterns = [
    ....
    path('check-user-online/', check_user_online, name='check_user_online'),
]



<script>
  setInterval(checkUserOnline, 5000); // every 5 seconds

  var checkUserOnline = function() {
    $.ajax({
        type: 'POST',
        url: '{% url "check_user_online" %}',
        success: function(response) {
          if(response['is_online']) {
             // do something if user online
          }else if(response['is_online'] == false) {
             // do somehitng if user offline
          }else {
             // do something if 'null'
          }
        }
    });
  }
</script>