Previous MSP is holding client’s site hostage

The MSP I work for has just taken over a client, whose previous managed service provider is not giving admin access to their WordPress. My client does have access to WordPress, but not admin access. I do have access to the DNS.

The client has already contacted lawyers regarding this but it's likely to be a long and drawn out affair.
Is there anything we can do to get access to their WordPress installation so we can transfer them over to us?