First of all, the question which mail server will receive emails for that domain is determined by the MX records in the DNS settings. OTOH, if you want your sending server to be recognized without problems, it should have correct reverse DNS set up, otherwise many mail servers around the world will not accept incoming email from it. This effectively means that the PTR record should resolve to a host name which, if resolved itself, points to the same IP address.
Having said all of this: if you are not a server admin, I would seriously recommend AGAINST you setting up a mail server for anybody. Running a mail server is not something you do in your spare time. It requires quite a bit of knowledge, skill and time to keep it up to date and configured in such a manner that it performs and doesn't get compromised. Rather get them a hosted mail server somewhere else.
Of course, if you are willing to put the time in (and hopefully you are able to bill your client for it), then by all means go ahead. Pick your favourite MTA and I am sure all of us here will be able to help you get it set up properly and safely.