There are several approaches you could take here, Raven. The easiest would be to simply give you access to his mail account, assuming he was using IMAP and all the mail is still online. However, his daughter may not want to do that, if it was an email account used for lots of other things too.
Next easiest option would be to forward all the relevant emails to you - note that I said “easiest” - not necessarily least time consuming!
There are a bunch of other options as well, but approximately how many emails are we talking about here? And were all the relevant emails sorted into folders (which could be online or offline - just like Mac Mail)?