Does she know anything about this old boyfriend? Are they on Facebook? Is there any contact or information with mutual friends? Or is she just trying to bombard the guy with high hopes to spark an old romance? Is he receptive? Does she know he's receptive? Is he married or in a rel@tionship? Is she planning on doing this completely blind without knowing anything?
I think if there is no other way to contact this person, ringing the bell is not the worst thing in the world. How else can you establish contact? There are many who have yet to be in the modern era, so old school contact may be the only way. I would encourage her to contact him by another means first. If she can't make use of Facebook, we know she has the guy's address, so send a card. If she is going to show up on this guy's porch willy-nilly, I think it's a fine gesture to bring a gift, and she should be prepared to not stay long, and leave her personal contact information, and do all this in a way that does not suggest romance unless she knows for a fact he is equally interested; otherwise, it's creepy and stalky.
Thirty years is a long time. This could land your relative in an era that porch calls were once very acceptable and normal, where today this might come across as creepy, even for the "old folks," so showing up unannounced may not be the best idea.