Well, "the developer" will presumably be a lab souping it in a strongly compensating developer - they often cannot do anything about it unless you pay for having a entire batch run at a extended development time. But even if they don't, a two stop underexposure won't look that horrible in the developers they use - many will nod friendly to your request, do nothing and you'll still be as satisfied (or frustrated) with the results as if they had indeed pushed. In general, pushing will save the highlights but boost the contrast, which isn't always that much better than having a underexposed film with manageable contrast. Either way, you should expect to lose the already underexposed frames while you will gain some others that you'd otherwise have overexposed. The bulk will still be printable, maybe a bit more dark and dramatic.
If you can do it yourself, a two stage developer would give it enough of a boost to rescue it entirely...
