Parse - Get object from pointer on beforeSave?

To answer your question directly, you can use Parse.Query.get() or Parse.Object.fetch() to retrieve the object.

I'm assuming that the problem you see is that the object saves but the push notification isn't happening. The cause is that you're not waiting for the get() to complete before calling response.success() and returning.

Here's a couple ways to reconcile that:

Your existing code but with response.success() moved up: Parse.Cloud.beforeSave("Request", function(request, response) {

    var userQuery = new Parse.Query("User")

    userQuery.get(request.object.get("to").id, {
        success: function (user) {
            console.log("user: ", user);
            var installationQuery = new Parse.Query("Installation");
            installationQuery.equalTo("user", user);

            Parse.Push.send({
                where : installationQuery,
                data : {
                    alert : "HEllo"
                },
                success : function() {
                    response.success();
                },
                error : function() {
                    console.log("error finding installation: " + error);
                }
            });
        },
        error : function (error) {
            console.log("ERRRRRRRRRR");
        }
    });
});

Simplified with Promises

Parse.Cloud.beforeSave("Request", function(request, response) {
    request.object.get("to").fetch().then(function(user) {
        var installationQuery = new Parse.Query("Installation");
        installationQuery.equalTo("user", user);

        return Parse.Push.send({
            where : installationQuery,
            data : {
                alert : "HEllo"
            }
        });
    }).then(function() {
        response.success();
    }, response.error);
});

Further simplified. If you're not using the data within user, you shouldn't need to fetch it just to pass a pointer to a query.

Parse.Cloud.beforeSave("Request", function(request, response) {
    var installationQuery = new Parse.Query("Installation");
    installationQuery.equalTo("user", request.object.get("to"));

    return Parse.Push.send({
        where : installationQuery,
        data : {
            alert : "HEllo"
        }
    }).then(function() {
        response.success();
    }, response.error);
});