Q&A - Stopping a roof valley leak

Hello roofing experts. I'm coming here to see if they I could please get some advise from those who know. Please excuse me if I don't use the correct lingo or say anything that might offend, I'm pretty uneducated in the ways of building and construction so that's why I'm seeking expert opinions Little background first. I live in the Phoenix AZ area and have a two story tile (cement or ceramic, not sure what's it made out of) roof home that is 15 years old. 3 years ago our roof developed a leak that was made known during a heavy monsoon. We had it fixed by a family friend that is roofer by trade. A couple months later it happens again. So our friend does more repairs but lets us know our house had bad underlayment and he had to replace a larger area this time to ensure it would work. He told us all the house's around us built during the housing boom had inadequate underlayment. 2 more years go by and this last summer during a heavy monsoon storm a new leak happened in a different room. So this has got me wanting to look into getting the whole roof redone with new underlayment. So I guess I'm asking how to make sure I hire a good company that will not rip me off. That family friend who fixed it the first time recommended I need to have Fontana G40 put down and he also said to make sure they are using new materials and not seconds or left overs. What are some things I can do as a homeowner to ensure I get quality work at a fair price. click https://www.goldentreeroofing.com/annandale-va.php Thanks in advance.