You can mention him/her in your cover letter, but I wouldn't recommend using him/her as a referee. He/She might be able to contact HR and tell them about your interest in the firm, or ask his/her friends who work there to recommend you, but when it comes to the application, I was advised against using former emloyees as referees.

Yes, if the alumn recently left the Firm and still knows the responsible colleages in recruiting, he can easily reach out to them (or to the partner in charge of recruiting for the specific office). Have seen this quite often (and have also done it myself).

Cheers, Sidi

(edited)

They won't be able to make an official referral - but of course, if they still have friends at the firm, they can recommend you through those channels. The more senior, the better - and if they have a relationship with someone in recruiting specifically, this can also be beneficial.

Hope this helps!