"imperial" porter question

A friend ran into a few problems on a porter (1.080 SG). He pitched 2 tubes of yeast (one Wyeast and one White Labs) and it started fermentation well but stalled out at 1.033. I had him add a little yeast energizer - it restarted and dropped to 1.028 and still is fermenting very slowly. 70% attenuation should get him down to 1.024 and that still seems sweet. 2 questions: 1) should he be patient and wait, or try more energizer or more yeast or ????? 2) if the yeast have stalled out, what should he do when he bottles? At the current pace the bottle might eventually charge. Adding priming sugar might just make a sweeter flat beer. I read in a winning MHBA recipe adding a little 2024 pilsen yeast at bottling - is this one possibility, or dangerous with that much available sugar?