Hi,
They can try Group VPN which is probably the closest to DMVPN, but your need Junos 12.1 or later
http://www.juniper.net/documentation/en_US/junos12.1x46/topics/concept/vpn-security-group-cisco-get-vpn-interoperability-understanding.html
There is also AutoVPN:
http://www.juniper.net/documentation/en_US/junos12.1x46/topics/task/configuration/security-autovpn-configuration-overview.html
Not sure what type of firewall your customer use, but if they want to go separate tunnel for each site, (500 VPNs) then they probably need to look at the SRX-3400 or 3600 series
HTH