Neither, assigning an account to jailshell would have no impact on php's curl functionality. I would look at the phpinfo to verify that curl is in fact included with your php configuration and if it is then I would stick a simple curl example page on their site to show them that it is working.
I find sometimes that developers who cannot get something to work automatically assume that its php's fault and not the code, so giving them a working example on the same domain usually takes that off the table.