Giveup, it's very much up to the individual. Personally, I have used an agency to do it (we have been offered a deal through work where the company takes 20% instead of the usual 25%). They have recovered PPI for me and I have gladly paid their fees - I don't have time or inclination to faff about chasing up the banks to find out what's happening, I prefer that someone else does that for me. They know the right terminology etc and they keep on the case for me and know when the time is right to take next steps.
I've very happy to let someone do it for me - it's one less thing I have to worry about in my busy life.
No company can "take her to the cleaners" - the fee is a percentage of the amount recovered, and is known in advance. She needs to shop around and not to be beguiled by companies who email or text canvassing for business.
It's not dodgy - it just suits some people better than all the letter-writing and phoning which is involved if you do it yourself. If she wants to do it that way, just let her get on with it. I'm happy doing it that way.