My client wants to control the member accounts (username/email/password etc) by their own API. So essentially when they update their own database the EE database would update automatically.
Any way this can be achieved? If so what does their API have to include?
It's certainly possible, but you would have to write your own API (in the form of an ExpressionEngine module) for their main system to access. EE doesn't have any built in web API to do this sort of thing (unless you already have access to the database directly, which might be easier).
Is EE in the same DB or an external one?
If they are both mySQL you could simply write a trigger on UPDATE or INSERT into their database that will then update or insert into the EE database. You can skip all the API stuff and let the DB handle it.
