Edit.cshtml

@model EditUserModel

@using (Html.BeginForm()) 
{ 
  <div>
    @Html.HiddenFor(m => m.Id)
    @Html.TextBoxFor(m => m.FirstName)
    @Html.TextBoxFor(m => m.LastName)
    @Html.DropDownListFor(m => m.CountryId, Model.Countries)
    
    <input type="submit" />
  </div>
}

EditUserModel.cs

class EditUserModel
{
    public string Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string CountryId { get; set; }
    public ICollection<SelectListItem> Countries { get; set; }
}

EditUserModelBuilder

class EditUserModelBuilder : IModelBuilder<EditUserModel, User>
{
    readonly ISession session;

    public EditUserModelBuilder(ISession session)
    {
        this.session = session;
    }

    public EditUserModel CreateFrom(User user)
    {
        var model = new EditUserModel();
        model.Id = user.FirstName;
        model.FirstName = user.FirstName;
        model.LastName = user.LastName;
        model.Country = user.Country.Id;
        model.Countries = GetCountries();
        return model;
    }

    public EditUserModel Rebuild(EditUserModel model)
    {
        model.Countries = GetCountries();
        return model;
    }

    public User ApplyChanges(EditUserModel model)
    {
        var user = string.IsNullOrEmpty(model.Id) ? new User() : session.Find<User>(model.Id); 
        session.Store(user);

        user.FirstName = model.FirstName;
        user.LastName = model.LastName;
        user.Country = session.Find<Country>(model.CountryId);
        return user;
    }

    ICollection<SelectListItem> GetCountries()
    {
        var countries = session.FindAll<Country>();
        return countries.Select(c => new SelectListItem { Value = c.Id, Text = c.Name }).ToList();
    }
}

IModelBuilder.cs

interface IModelBuilder<TViewModel, TEntity>
{
    TViewModel CreateFrom(TEntity entity);
    TViewModel Rebuild(TViewModel model);
    TEntity ApplyChanges(TViewModel viewModel); 
}

UserController.cs

class UserController : Controller
{
    IModelBuilder<EditUserModel, User> builder = new EditUserModelBuilder();

    public ActionResult Edit(string id)
    {
        var user = session.Find<User>(id) ?? new User();
        return View(model, builder.CreateFrom(user));
    }

    [HttpPost]
    public ActionResult Edit(EditUserModel model)
    {
        if (!ModelState.IsValid)
        {
            return View(builder.Rebuild(model);       
        }
        
        builder.ApplyChanges(model);
        session.SaveChanges();
        return RedirectToAction("Index");
    }
}

Looks great :)

There is another mistake in your IModelBuilder.cs line 4. It should be TEditUserModel