I am not really sure of how the licensing works. But if you can get a copy of Client version of MQ for your use, you could connect to MQ server installed in your company without any issues. The client company does not need to get a license for a server version if it doesnt use or install one. Since they only need a client, they could just get a copy(as i said before, either licensed or trial version, if that suits) and use to connect your server(MQ).
Cheers
KK