About 25 years ago when I was showing my first Cocker, golden boy Josh, I had a couple of enquiries about using him at stud. I was flattered but puzzled. Why would people want to use Josh, owned by a complete novice with zero experience of dog breeding, as a stud dog? He was doing well at shows but wasn't outstanding, and his sire and a litter brother were at stud, both owned by experienced breeders, so I just said a polite no and left it at that.
After 20 years of not showing I then started to show blue roan and tan Louis in 2010. A breeder/exhibitor (not a COLer) asked me repeatedly if I would be interested in using him on some of her bitches. She knew Louis's pedigree extremely well so knew how weak it is (in showing terms) on the dam's side. Again I said no and eventually she stopped asking. I've always assumed it was just his blue roan and tan gene that she was interested in.
So I wonder if the lady who asked you has made enquiries with other Cocker dog owners too? No disrespect to Charlie or you at all, but as Jane has said, the obvious thing to do would be to use Charlie's sire or another proven dog from similar lines.
As for behaviour changes, when Josh was young he had a very good Cocker friend called Smartie, who was always beating us at shows

but the 2 of them liked to play and chase around when not in the showring and became very good friends when Josh stayed with Smartie's mum for a week when I was on holiday. Smartie later became a stud dog and the first time he met Josh after that he had a real change in attitude and Josh's friendly greeting was met with growling and then biting when Josh didn't take the hint that his friend no longer liked him. So behaviour changes can happen too.
For me it was nice to think that somebody liked my boys enough to want to use them at stud, but I doubted their reasons for wanting to use them and I knew that I didn't have the knowledge or experience to cope. Also, there are so many Cocker puppies being bred that it's not always easy for breeders to find good homes and I didn't feel that Josh or Louis had anything special to bring to the breed so I couldn't personally justify using them.
Just my thoughts and experiences, but hope they're helpful
