You shouldn't have any trouble making those enlargements with your files, providing the quality of your files is impeccable. If they aren't tack sharp or if there are any other flaws, those flaws will only be magnified. But if you're starting with a very high quality source, you can go quite large. I once made a 400cm tall print from a Canon 1Ds file (11 megapixels) and it turned out quite well.
I would echo the advice about working closely with the printer and you can do that even if they're in another country. The printer typically knows exactly what they need to produce good results. Talk to them directly. Don't rely on your client to be the middleman.
If at all possible, help your client choose the printer. Use the internet to find a list of printers close to the area where the print will end up and call them. Even in a short phone call, you can often get a good handle on whether or not the printer knows what they're doing. Call several, but give your client a finalist or two to go visit.
I do this kind of work regularly and in every case, I've provided the printer with a file that I have enlarged myself. I go back to the original RAW file and do a new conversion, turning off all sharpening. If you don't, the enlargement may have ugly halos. I save this conversion as a 16-bit TIFF and run it through an enlargement program called SizeFixer. It's slow – even on a fast machine, the enlargement can take a few hours – but it contains profiles for specific digital cameras. Use the profile for your camera and run the enlargement at the maximum quality setting to the file size that the printer specified.
If the printer is going to apply the final sharpening, I give them the output from SizeFixer. If they're looking for more of a final file to print from, I will over enlarge through SizeFixer (if the printer asks for 240 dpi, I'll enlarge to 300 or 360 dpi), apply some sharpening, and then reduce it to 240 dpi.
But, I can't stress this enough, try to work directly with the printer to ensure you're giving them the best file for the job. You're going to be judged on the final output, so you and the printer need to work as a team to ensure the client is happy.