Since we don't know details yet, I can only speak in vagary, but we have had good luck in Los Angeles doing the following things:
0- I put this one ahead of number one, because it is the MOST important. Find out the variety. There are MANY varieties that will do will in our mild climates. However, most don't. MrTree was way off when he said you can't grow a peach here. You can. I know LA zones are very similar to SD, so my experiences will be similar to what you're dealing with. Now if you CAN find out the variety then we can look up how many chill hours it needs and you can find out if that type of peach is matched to a good zone for it. If it isn't in an area that gives enough chill hours for that variety, then the rest is all a gamble.
1- Get them on a good fertilizing regimen. Practically no home owners in SoCal fert their fruit trees and many of them SHOULD be. Find out how long they've lived on the property/had the tree, then follow up with the fert question
2- Proper watering schedule. Ask them what they do. I bet it's all wrong for the tree.
3- IS there lawn around the tree? Get it the F away from it! Add lots of mulch, too.
4- Prune? Yes. It is not THE answer, but it is part of the solution.
5- Thin the fruit. scubadude is right about this one. For some of our clients we will remove as much as 90% of all the fruit on the tree. I don't want them there when we do the work because they try to get less removed. The client has to be on board. It's usually an easy sell because I tell them, "you can have one thousand useless fruit or 100 good fruit...what's it gonna be."
There likely isn't a quick one step solution that you can apply. You don't know the history and my guess is that the clients don't either. I take the approach of "we don't know exactly what one factor we need to tweak, so lets tweak EVERYTHING and work from there."
love
nick