There are a number of ways you can go. First, if you need a new roof, get it before it causes you other problems! Do take pictures BEFORE you do the job. Check around the house to see if there are any left-over shingles from the 5 year old job. Can you find out from the previous owners who did the job? Do they have the paperwork? (Make copies.) Save some of the ‘old’ shingles too in case you need some proof. See if your roofer can get you a shingle description / brand, in writing, to you for your records. After that, then pursue your remedies.
You can check with neighbors to see if anyone else in the neighborhood either had their roof done when yours was done, or if they know who did your roof. You can also check around with a few roofers or contractors to see if they know of any manufacturers that had made defective roofing products in the past 10 years (not just 5). Also, you can check with the the BBB to see if they know of roofers who installed defective shingles and never made ‘good’ on them. And lastly, you can check with the CPSC (Consumer Product Safety Commission). They have a WEB site but I don’t know what it is. (Search the WEB for the name I just gave you.)
In closing, your inspector may not necessarily be correct. If there are other homes in your neighborhood that were built by the same contractor, see if they’ve had similiar problems. You see, your bad shingles can be from other causes. Such as: Poorly vented attic, water, bad underlayment / paper, poor shingles (not necessarily ‘defective’ shingles), and it’s not uncommon for a 15 year shingle to last only 5 years if any of the aforementioned conditions exist, plus, lots of direct sun (as with a south-facing roof). At this point, I wish you well.
If you have any more questions, post up. My best to ya and hope this helps.