Hi Fiona
I agree with Pam - I've never heard of an 18 week old Cocker coming into season, just occasionally I hear of a 5 or 6 months old one having their first season but that's fairly unusual - normally it's a bit later.
Re your Dad taking his Cocker out in season, he can carry on lead walking her for a week or so but I would advise him to keep her at home during the time when she will be most attractive to other dogs (which varies but is usually between days 10-15) As someone who walks male dogs, I don't appreciate it when owners walk bitches in full season where I take my dogs - it doesn't make for an enjoyable walk

It's maybe ok if he keeps to street walking but not if he takes her anywhere like a park/field where other dogs are exercised.
Jane